In an undated photo released on November 28, North Korea leader Kim Jong Un is seen on a field trip to see the airwomen of the KPA Air and Anti-Air Force. North Korean Newspaper Rodong Sinmun reported Kim "guided a flight drill of pursuit airwomen of the KPA Air and Anti-Air Force. He went out to an airport's runway to learn about the plan for solo take-off and landing drill by women pilots of pursuit planes and guide their flight."
A picture released by the North Korean Central News Agency shows Kim appearing without his cane at an event with military commanders in Pyongyang on Tuesday, November 4. Kim, who recently disappeared from public view for about six weeks, had a cyst removed from his right ankle, a lawmaker told CNN.
Kim is seen walking with a cane in this image released Thursday, October 30, by the state-run Korean Central News Agency.
Kim talks with officials during an airmen inspection.
Kim sits in the pilot's seat of a fighter jet during the inspection.
Kim sits at a desk during the inspection.
This undated photo, released Tuesday, October 14, by the KCNA, shows Kim inspecting a housing complex in Pyongyang, North Korea. International speculation about Kim went into overdrive after he failed to attend events on Friday, October 10, the 65th anniversary of the Workers' Party. He hadn't been seen in public since he reportedly attended a concert with his wife on September 3.
A picture released by the KCNA shows Kim and his wife watching a performance by the Moranbong Band on Wednesday, September 3, in Pyongyang.
Kim tours a front-line military unit in this image released Wednesday, July 16, by the KCNA.
Kim poses for a photo as he oversees a tactical rocket-firing drill in June.
Kim watches a tactical rocket-firing drill in June.
A North Korean soldier patrols the bank of the Yalu River, which separates the North Korean town of Sinuiju from the Chinese border town of Dandong, on Saturday, April 26.
A soldier looks through binoculars inside a sentry post along the bank of the Yalu River on Monday, April 7.
In this photo released Thursday, April 24, by the Korean Central News Agency, Kim smiles with female soldiers after inspecting a rocket-launching drill at an undisclosed location.
A picture released Tuesday, March 18, by the KCNA shows Kim attending a shooting practice at a military academy in Pyongyang.
A North Korean soldier uses binoculars on Thursday, February 6, to look at South Korea from the border village of Panmunjom, which has separated the two Koreas since the Korean War.
A North Korean soldier kicks a pole along the banks of the Yalu River on Tuesday, February 4.
A photo released by the KCNA on Thursday, January 23, shows the North Korean leader inspecting an army unit during a winter drill.
Kim inspects the command of an army unit in this undated photo released Sunday, January 12, by the KCNA.
Kim visits an army unit in this undated photo.
A picture released by the KCNA on Wednesday, December 25, shows Kim visiting an army unit near the western port city of Nampo.
Kim inspects a military factory in this undated picture released by the KCNA in May 2013.
Kim visits the Ministry of People's Security in 2013 as part of the country's May Day celebrations.
A North Korean soldier, near Sinuiju, gestures to stop photographers from taking photos in April 2013.
North Korean soldiers gather by the docks in Sinuiju, near the Chinese border, in April 2013.
North Korean soldiers patrol near the Yalu River in April 2013.
Kim is briefed by his generals in this undated photo. On the wall is a map titled "Plan for the strategic forces to target mainland U.S."
Kim works during a briefing in this undated photo.
In this KCNA photo, Kim inspects naval drills at an undisclosed location on North Korea's east coast in March 2013.
Kim, with North Korean soldiers, makes his way to an observation post in March 2013.
Kim uses a pair of binoculars to look south from the Jangjae Islet Defense Detachment, near South Korea's Taeyonphyong Island, in March 2013.
Kim is greeted by a soldier's family as he inspects the Jangjae Islet Defense Detachment in March 2013.
Kim is surrounded by soldiers during a visit to the Mu Islet Hero Defense Detachment, also near Taeyonphyong Island, in March 2013.
Kim arrives at Jangjae Islet by boat to meet with soldiers of the Jangjae Islet Defense Detachment in March 2013.
Soldiers in the North Korean army train at an undisclosed location in March 2013.
In a photo released by the official North Korean news agency in December 2012, Kim celebrates a rocket's launch with staff from the satellite control center in Pyongyang.
Kim, center, poses in this undated picture released by North Korea's official news agency in November 2012.
Kim visits the Rungna People's Pleasure Ground, under construction in Pyongyang, in a photo released in July 2012 by the KCNA.
A crowd watches as statues of North Korean founder Kim Il Sung and his son Kim Jong Il are unveiled during a ceremony in Pyongyang in April 2012.
A North Korean soldier stands guard in front of an UNHA III rocket at the Tangachai-ri Space Center in April 2012.
In April 2012, Pyongyang launched a long-range rocket that broke apart and fell into the sea. Here, the UNHA III rocket is pictured on its launch pad in Tang Chung Ri, North Korea.
A closer look at the UNHA III rocket on its launch pad in Tang Chung Ri, North Korea.
A military vehicle participates in a parade in Pyongyang in April 2012.
North Koreans wave flags in front of portraits of Kim Il Sung, left, and Kim Jong Il during celebrations to mark the 100th birth anniversary of Kim Il Sung in Pyongyang in April 2012.
North Korean soldiers relax at the end of an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012.
Kim Jong Un applauds as he watches a military parade in Pyongyang in April 2012.
A North Korean soldier stands on a balcony in Pyongyang in April 2012.
North Korean soldiers march during a military parade in Pyongyang in April 2012.
Soldiers board a bus outside a theater in Pyongyang in April 2012.
North Korean performers sit below a screen showing images of leader Kim Jong Un in Pyongyang in April 2012.
North Korean soldiers salute during a military parade in Pyongyang in April 2012.
North Korean soldiers listen to a speech during an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012.
Members of a North Korean military band gather following an official ceremony at the Kim Il Sung stadium in Pyongyang in April 2012.
North Korean military personnel watch a performance in Pyongyang in April 2012.
A North Korean controller is seen along the railway line between the Pyongyang and North Pyongan provinces in April 2012.
A North Korean military honor guard stands at attention at Pyongyang's airport in May 2001.
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
Kim Jong Un and North Korea's military
STORY HIGHLIGHTS
- NEW: North Korea blocks South Korean workers from jointly run industrial complex
- The U.S. is ''fully prepared" to defend itself, Kerry says
- U.N. Secretary-General Ban Ki-moon "deeply troubled" about North Korea's statements
- The North says it is adapting and restarting facilities at its main nuclear complex
Are you from South or North Korea? Concerned about the latest crisis? Send us your thoughts.
(CNN) -- The United States will not accept North Korea as a "nuclear state," Secretary of State John Kerry warned on Tuesday, just hours after Pyongyang announced plans to restart a nuclear reactor it shut down five years ago.
North Korea's decision comes as tensions on the Korean peninsula escalate over Kim Jong Un's threats to wage war against the United States and South Korea.
"The bottom line is simply that what Kim Jong Un is choosing to do is provocative. It is dangerous, reckless. The United States will not accept the DPRK (Democratic People's Republic of Korea) as a nuclear state," Kerry said during a joint briefing in Washington with South Korea Foreign Minister Yun Byung-se.
"And I reiterate again the United States will do what is necessary to defend ourselves and defend our allies, Korea and Japan. We are fully prepared and capable of doing so, and I think the DPRK understands that."
Kerry: U.S. won't accept a nuclear state
North Korea's threats 'gone too far'
North Korea's advantage explained
Analyst: Don't panic over NK nuke plan
North Korea's declaration that it would reopen the reactor demonstrates Kim's commitment to the country's nuclear weapons program that the international community has tried to persuade it to abandon.
The North's state-run Korean Central News Agency, KCNA, reported that the reclusive state's atomic energy department intends to "readjust and restart all the nuclear facilities" at its main nuclear complex, in Yongbyon.
Those facilities include a uranium enrichment facility and a reactor that was "mothballed and disabled" under an agreement reached in October 2007 during talks among North Korea, the United States and four other nations, KCNA said.
The announcement was followed by a plea for calm from U.N. Secretary-General Ban Ki-moon, who is South Korean.
North Korean defectors return rhetorical fire
"The current crisis has already gone too far," he said in a statement from Andorra. "Nuclear threats are not a game. Aggressive rhetoric and military posturing only result in counteractions, and fuel fear and instability.
"Things must begin to calm down, as this situation, made worse by the lack of communication, could lead down a path that nobody should want to follow."
Ban said dialogue and negotiations are "the only way to resolve the current crisis."
The tensions on the Korean Peninsula have led Pyongyang to sever a key military hot line with Seoul and declare void the 1953 armistice that stopped the Korean War.
The United States has made a show of its military strength amid annual training exercises with South Korea, flying B-2 stealth bombers capable of carrying conventional or nuclear weapons, Cold War-era B-52s and F-22 Raptor stealth fighters over South Korea.
On Tuesday, the United States positioned a second destroyer -- the USS Decatur -- near the Korean peninsula, a defense official said on condition of anonymity. The official was not authorized to release details to the media.
A day earlier, the U.S Navy moved a warship and a sea-based radar platform closer to the North Korean coast in order to monitor that country's military moves, including possible new missile launches, the Defense Department said Monday.
North Korea on Wednesday also blocked more than 480 South Korean workers from entering the jointly run Kaesong Industrial Complex, which sits on the North Korea side of the border, according to the South Korean Unification Ministry.
"North Korea said today that it is only approving people to go back to South Korea and not approving entrance" to the complex, said Kim Hyung-seok, a spokesman for the ministry. He urged the North to resume letting workers enter.
S. Koreans barred from N. Korean complex
Analyst: We can't talk about it as bluff
South Korea returns rhetorical fire
Life in North Korea through a lens
There are 861 South Koreans inside Kaesong at the moment, the ministry said, and 446 have registered to leave.
The symbol of an era of better relations between Seoul and Pyongyang, Kaesong is a joint economic cooperation zone between the two Koreas where scores of South Korean companies operate. It also is an important source of hard currency for the North.
The move comes after threats in recent days by Pyongyang to shut down the industrial complex.
Seoul, meanwhile, on Monday warned that any provocative moves from North Korea would trigger a strong response "without any political considerations."
U.S. moves warship, sea-based radar to watch North Korea
Murky motivation
The motivation behind the North's announcement Tuesday on the nuclear facilities was unclear, said Ramesh Thakur, director of the Center for Nuclear Nonproliferation and Disarmament at Australian National University in Canberra, suggesting that it was unlikely to make a big difference militarily for the country, which is already believed to have four to 10 nuclear weapons.
The North Koreans may be hoping to use the move as a bargaining chip in any future talks, he said, or it could be an attempt by the country's young leader to shore up support domestically.
"It's just a very murky situation," Thakur said. "The danger is that we can misread one another and end up with a conflict that no one wants."
China, a key North Korean ally, expressed regret over Pyongyang's announcement about the reactor.
"China has consistently advocated denuclearization on the peninsula and maintaining peace and stability in the region," Chinese Foreign Ministry Spokesman Hong Lei said Tuesday at a regular news briefing.
Japanese Chief Cabinet Secretary Yoshihide Suga said the move would need to be dealt with in a serious manner, noting that it breached the North's previous commitments.
On Tuesday, Kerry refused to speculate about North Korea's intentions or what its strategy may be with regard to its plans to reopen its reactor.
"We've heard an extraordinary amount of unacceptable rhetoric from the North Korean government in the last days. So let me be perfectly clear here today: The United States will defend and protect ourselves, and our treaty ally, the Republic of Korea," he said.
Kerry reiterated the U.S. policy with regard to North Korea, saying the United States believes there is "a very simple way" for Pyongyang to end the sanctions by ending its nuclear ambitions.
Kerry was scheduled to visit Seoul next week, while South Korea's president was due in Washington for talks with President Barack Obama.
South Korea warns North of 'strong response' to any attack
A torrent of threats
The North's latest declaration comes after a stream of verbal attacks against South Korea and the United States in recent weeks, including the threat of a nuclear strike.
Pyongyang's angry words appear to have been fueled by the recent joint military exercises by the United States and South Korea in the region, as well as tougher U.N. sanctions in response to North Korea's latest nuclear test in February.
Much of the bellicose rhetoric, analysts say, isn't matched by the country's military capabilities.
Nuclear weapons: Who has what?
The North's announcement Tuesday follows a new strategic line "on simultaneously pushing forward economic construction and the building of the nuclear armed force." It was announced Sunday during a meeting of a key committee of the ruling Workers' Party of Korea headed by Kim Jong Un.
The work of adapting and restarting the nuclear facilities "will be put into practice without delay," KCNA said.
The measures would help solve "the acute shortage of electricity," as well as improving the "quality and quantity" of the country's nuclear arsenal, it said.
Threats of annihilation normal for South Koreans
Yongbyon's back story
In June 2008, the usually secretive North Korean government made a public show of destroying the cooling tower of the Yongbyon reactor to demonstrate its compliance with a deal to disable its nuclear facilities.
But two months later, as its then-leader, Kim Jong Il, balked at U.S. demands for close inspections of its nuclear facilities, the North started to express second thoughts.
It said it was suspending the disabling of its nuclear facilities and considering steps to restore the facilities at Yongbyon "to their original state."
In November 2009, it announced it was reprocessing nuclear fuel rods as part of measures to resume activities at Yongbyon. It noted success in turning the plutonium it had extracted into weapons-grade material.
OPINION: Why Kim Jong Un is not crazy
CNN's K.J. Kwon in Seoul, Tim Schwarz in Hong Kong, Dayu Zhang in Beijing, Yoko Wakatsuki in Tokyo and Barbara Starr and Elise Labott in Washington contributed to this report.