Seoul's National Intelligence Service (NIS) told South Korean lawmakers at a closed door parliamentary session that it has detected signs of North Korea preparing for another nuclear test at its Punggye-ri underground test site.
North Korea's last nuclear test came in September, when it detonated what it claimed was a miniaturized nuclear warhead small enough to be mounted on a missile, though analysts say the claim is nearly impossible to verify.
Kim Byung-kee, a lawmaker of South Korea's ruling Democratic Party (DP) said the NIS reported that North Korea "has completed its preparation to carry out a nuclear test at Tunnel 2 and Tunnel 3 of the Punggye-ri nuclear test site."
He added the NIS said it had detected activity suggesting Tunnel 4 was being prepared for more construction work after excavation work was halted last year.
Speaking to CNN's Will Ripley Monday, North Korean officials said the US faces strong punishment for conducting its current US-South Korea military games on the peninsula.
The 10-day Ulchi Freedom Guardian exercises, which began August 21, are conducted annually and touted by South Korea and the United States as defensive in nature.
But North Korean officials told Ripley this year's exercises come at the "worst possible moment" of tension.
If there is an escalation, with any "catastrophic consequences, the Americans would be wholly responsible," the officials said.
Meanwhile North Korea has continued to test a variety of missiles, including a triple launch on Saturday
The three missiles fired by Pyongyang in that test were short-range Scud missiles, Gen. Jeong Kyeong-doo, chairman of the Joint Chiefs of Staff, said Monday.
South Korea's presidential office said on Saturday that officials initially believed North Korea could have fired 300mm rocket artillery from a multiple rocket launcher.
A South Korean official said US and South Korean authorities were continuing their analysis of Saturday's launch, which saw two missiles travel 250 kilometers (155 miles) over the ocean and another explode shortly after liftoff.
It's the first missile test since North Korea launched successfully launched two intercontinental ballistic missiles in July which is said could reach as far as the US mainland.
North Korea's war games
Saturday's missile test came just after North Korea conducted large-scale military drills on Friday.
North Korean state media showed leader Kim Jong Un overseeing a special forces operation which simulated strikes on South Korean islands, CNN's Ripley reported from Pyongyang.
Saturday's missile tests came just days after US Secretary of State Rex Tillerson said North Korea was showing signs of restraint in its standoff with Washington and Seoul.
But barely a day later after Tillerson spoke, photographs emerged showing Kim inspecting missile and missile-fuel components.
Analysts said the photos were a sign from North Korea to the world, to show their solid-fuel missile program is improving at a steady rate.
Solid-fuel missiles are faster and easier to deploy, and harder to detect because there's less to be done in terms of launch preparation. All US and Russian ballistic missiles are solid-fuel models.
A statement on the state-run Korean Central News Agency on Saturday said Kim Jong Un had turned North Korea into a "matchless nuclear weapons state in a short span of time."
On Sunday state media discounted Tillerson's "restraint" statement and one from US President Donald Trump later in the week that Kim "is starting to respect us."
The article, from state newspaper Rodong Sinmun, said US leaders were trying to fool North Korea into giving up its nuclear weapons but they would not succeed.
Pyongyang would "sharpen its just nuclear treasured sword in its hand and defend independence and justice with nukes," the article said.
What North Korea learns
Analysts said no matter what was launched on Saturday, or the result of the launch, it represents a win for the Kim regime in developing its missile program.
"North Korea gains information from every missile launch, whether it succeeds or whether it fails," said Bruce Bennett, senior analyst at the Rand Corp. think tank.
"If it fails, then North Korea discovers problems with either their basic design or with the way the particular failed missile was put together," Bennett said "If the missile succeeds, they confirm that they have a reasonable design and that at least one of a given production batch really did fly."
"The fact that one of the rockets failed makes clear that (North Korea) may have some production issues it needs to solve, but the other two suggest that the underlying technology is working," said David Wright, co-director of the Global Security Program at the Union of Concerned Scientists.
Air show axed
Also at the weekend, North Korea said it was canceling the Wonsan Air Show, which was scheduled for September.
The show, primarily a showcase for state carrier Air Koryo, was popular last year. Air Koryo was sanctioned by the US last year but continues to operate regular flights from China and Russia.
It's the second major tourist event North Korea has canceled this year. The Taedonggong Beer Festival slated for August was called off amid reports of North Korea's worst drought in 16 years.