(CNN) -- North and South Korean artillery batteries exchanged hundreds of shells across their western sea border Monday, a day after North Korea warned it was preparing to test another nuclear device.
About 100 of the 500 shells North Korea fired into the Yellow Sea strayed across the line separating the two rivals' territorial waters, the semiofficial South Korean news agency Yonhap reported. Yonhap quoted the South Korean Joint Chiefs of Staff as saying the South responded by firing about 300 shells into North Korean waters and dispatching fighter jets to the boundary, known as the Northern Limit Line.
North Korean offshore firing appeared to have resumed after a lull, Yonhap reported, citing a resident of Baekryong Island, which is close to the Northern Limit Line.
"Some (North Korean) artillery fire landed in (the) southern part of Northern Limit Line but in the water," a South Korean Ministry of Defense spokesman said. "We counter-fired over the Northern Limit Line."
When asked what South Korea fired back at, the defense spokesman said, "We are not shooting at North Korea, just shooting into the sea."
The United States, South Korea's leading ally, condemned the North Korean shelling from the White House and the Pentagon.
Washington is working "in close coordination" with South Korea and Japan, White House spokesman Jay Carney said, calling on North Korea "to refrain from actions that threaten regional peace and security."
And Defense Secretary Chuck Hagel told reporters at the Pentagon, "The provocation that the North Koreans have, once again, engaged in is dangerous, and it needs to stop."
China, the North's main patron, also expressed concern.
"The temperature is rising at present on the Korean Peninsula, and this worries us," Foreign Ministry spokesman Hong Lei told reporters in Beijing. "We hope that all sides can remain calm and exercise restraint."
The normally reclusive North took the unusual step of informing its neighbor of live-fire drills close in the heavily militarized western sea. Pyongyang sent a fax early Monday demanding that the South "control" its vessels in seven areas of the waterway near the Northern Limit Line.
According to Wee Yong-Sub, a vice spokesman for the South Korean Defense Ministry, the scheduled tests mark the first time -- in recent history, at least -- that the North has announced live-firing exercises above the maritime border.
"We consider such announcement as a hostile threat and so have activated crisis management operation in case of (military) provocation," he said. "We stress that we are fully prepared for all situations."
Victor Cha, a leading Korea analyst, told CNN that the North may be "posturing" for attention in hopes bringing Washington back to talks over its nuclear program -- or moving while the United States distracted by other global events.
"They could be learning from Crimea that while the United States is distracted, the North Koreans can try to change the playing field and maybe slant it in their direction by pushing it back to talks while the United States is focused on other issues," Cha said.
The two Koreans never signed a peace agreement after the 1950-53 war that also pitted the United States and China against each other. Cha called it a "clearly a good thing" that Pyongyang notified the South of its military exercise. But if Northern gunners ended up killing someone across the border, "then we're in a pretty bad situation."
"They are on a hair trigger, and because of the array of forces on the peninsula, you can get an action-reaction dynamic that escalates fairly quickly," he said. "That's something we want to avoid, of course."
North Korea said Sunday that it "would not rule out" a new nuclear test as it defended its recent mid-range missile launch that triggered international condemnation.
"(We) would not rule out a new form of a nuclear test aimed at strengthening our nuclear deterrence," Pyongyang's Foreign Ministry said in a statement carried by the state-run KCNA news agency. "The U.S. had better ponder over this and stop acting rashly."
The statement did not specify what North Korea meant by a "new form" of test, and Wee said there are no immediate signs of nuclear tests being carried out by the North.
Last week, Pyongyang launched two medium-range ballistic missiles into the sea off its east coast, violating United Nations resolutions that prohibit Pyongyang from conducting such tests. The Security Council condemned the move and is considering an "appropriate response," said Luxembourg Ambassador Sylvie Lucas, the council's current president.
The military exercises are the latest provocation by the North and come after a maritime dispute last week was seemingly swiftly resolved. On Thursday, a North Korean fishing boat was seized after an alleged incursion into South Korean waters and returned with its three crew members the following day.
And while North Korea often upsets its neighbors by firing various rockets and missiles into the waters surrounding the Korean Peninsula, the country has at times engaged in more deadly military actions.
A multinational 2010 report indicated that the sinking of the South Korean navy warship Cheonan, which killed 46 sailors in the Yellow Sea, was the result of a a North Korean torpedo. Later that year, North Korean artillery attacks on Yeonpyeong Island in the Yellow Sea killed two South Korean marines in what Yonhap called "the first direct artillery attack on South Korean territory since the Korean War ended in an armistice" in 1953.
CNN's Jim Acosta contributed to this report.