South Korea's military says it's carefully monitoring North Korean nuclear and missile facilities after the country's spy agency told lawmakers that new activity was detected at a research center where the North presumably builds its long-range missiles targeting the US mainland.

Seoul's Defense Ministry spokeswoman Choi Hyun-soo said on Thursday the US and South Korean militaries are maintaining close intelligence coordination over developments at the North's missile research center in Pyongyang and also a separate long-range rocket site. She did not elaborate what the developments were.

But a lawmaker who attended a closed-door intelligence briefing told The Associated Press that National Intelligence Service director Suh Hoon said his agency was spotting increased vehicle movement at the Pyongyang facility. Suh during Tuesday's briefing said it was clear that vehicles were transporting supplies but avoided specific answers when lawmakers pressed him on what those would be for, the lawmaker said. The lawmaker requested anonymity because the information was sensitive.

Suh also told lawmakers that the North was restoring facilities at a rocket launch site in Tongchang-ri that it partially dismantled last year as part of disarmament steps, an assessment supported by private US reports based on satellite imagery. While the NIS believes the North has not produced plutonium for nuclear weapons in months, signs of use have been seen at the uranium-enrichment facility at the North's main nuclear complex in Yongbyon, the lawmaker said. The International Atomic Energy Agency provided a similar estimation in a recent report.

The revelations follow the talks last week between North Korean leader Kim Jong Un and President Donald Trump that collapsed over what the Americans said were North Korea's excessive demands for sanctions relief in exchange for a limited offer to partially shutter the Yongbyon site.

It wasn't immediately clear how the findings might affect the diplomacy. The US and North Korea accused each other of causing the breakdown of the summit in Vietnam, but both sides left the door open for future negotiations.

'We have an understanding about the (possible) North Korean intent behind these moves, but we cannot share it publicly,' said Choi from the Defense Ministry.

When asked about the reports on the Tongchang-ri site, Trump told reporters at the White House he would be 'very, very disappointed in Chairman Kim' if that were happening. He also said it was 'a very early report' and that 'we'll see what happens. We'll take a look. It will ultimately get solved.'

North Korea didn't immediately respond in its state media.

Experts say the missile research center in Pyongyang's Sanumdong area is where the North assembles its intercontinental ballistic missiles. The North conducted three flight tests of developmental ICBMs in 2017 that demonstrated the potential capability to reach the continental United States.

Following his last ICBM test in November 2017, North Korean leader Kim Jong Un declared his nuclear force as complete and initiated diplomacy with Washington and Seoul at the start of 2018. But experts say the North still needs to master a few remaining technologies, such as ensuring that the warhead survives the harsh conditions of atmospheric re-entry, to have functioning ICBMs.

Dismantling parts of the long-range rocket launch facility was among several steps the North took last year when it entered nuclear talks with the United States and South Korea. North Korea has carried out satellite launches at the site in recent years, resulting in U.N. sanctions over expert claims that they were disguised tests of banned missile technology.

Trump said Kim told him that North Korea would continue to suspend nuclear and missile tests while negotiations are underway, and South Korea and the US announced Sunday that they are eliminating massive springtime military drills and replacing them with smaller exercises in an effort to support the talks.

In early 2018, North Korea abruptly expressed its intention to deal away its weapons arsenal in return for security and economic benefits. The North has since unilaterally suspended nuclear and missile tests and dismantled its underground nuclear testing site without the presence of outside experts.

Its dismantling of parts of the Tongchang-ri facility occurred at the beginning of US-North Korea negotiations last year. Both the launch pad and engine test stand were in about the same condition since last August, according to a report by 38 North, a North Korea-focused website.

After a September summit with South Korean President Moon Jae-in in Pyongyang, North Korea's capital, Kim agreed to 'permanently shut down' the Tongchang-ri engine test ground and rocket launch pad with the participation of foreign experts.