SEOUL (Reuters) - North Korea may be planning a new missile test, South Korea’s spy agency told lawmakers on Thursday, after brisk activity was spotted at its research facilities, just days before U.S. President Donald Trump visits Seoul.

North Korean leader Kim Jong Un visits a cosmetics factory in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on October 28, 2017. KCNA/via REUTERS

Reclusive North Korea has carried out a series of nuclear and missile tests in defiance of U.N. Security Council resolutions, but has not launched any missiles since firing one over Japan on Sept. 15, the longest such lull this year.

However a flurry of activity including the movement of vehicles has been detected at the North’s missile research facilities in Pyongyang, where the most recent missile test was conducted, pointing to another possible launch, South Korea’s Intelligence Service said in a briefing to lawmakers.

It did not say how the activity was detected.

North Korea has made no secret of its plans to perfect a nuclear-tipped missile capable of reaching the U.S. mainland. It regularly threatens to destroy the United States and its “puppet”, South Korea.

“There is a possibility of a new missile launch given the active movement of vehicles around the missile research institute in Pyongyang. The North will constantly push for further nuclear tests going forward, and the miniaturization and diversification of warheads,” the intelligence agency said at the briefing.

The North’s nuclear testing site in the northwestern town of Punggye-ri could have been damaged by its sixth and largest nuclear test on Sept. 3, according to Kim Byung-kee, Yi Wan-young and Lee Tae-gyu, members of South Korea’s parliamentary intelligence committee.

The explosion triggered an aftershock within eight minutes and three additional shocks.

Japanese broadcaster TV Asahi, citing unnamed sources, said on Tuesday a tunnel at the test site collapsed after that explosion, possibly killing more than 200 people. Reuters has not been able to verify the report which North Korea on Thursday denounced as false and defamatory.

Pyongyang will likely detonate more devices as it tries to master the miniaturization of nuclear warheads to put atop missiles, the lawmakers said.

The third tunnel at the Punggye-ri complex remained ready for another test “at any time”, while construction had resumed at a fourth tunnel, making it unable to be used “for a considerable amount of time”, they added.

Trump is to visit five Asian nations in coming days for talks in which North Korea will be a major focus. The visit includes the North’s lone major ally, China, and U.S. allies Japan and South Korea, which have watched with increasing worry as Trump and North Korea have exchanged bellicose rhetoric.