There was activity at the Sanumdong site before President Donald Trump met with North Korean leader Kim Jong Un, during which the two leaders discussed the future of the latter's nuclear weapons program.
USA spy agencies said that new evidence - including satellite photos - reveal that the country is creating at least one liquid-fueled ICBM at a research facility outside Pyongyang, according to The Washington Post.
Imagery from the National Geospatial-Intelligence Agency suggests ongoing work on at least one Hwasong-15 ICBM at the Sanumdong plant, the Washington Post said.
After that meeting, the USA president declared North Korea was no longer a nuclear threat and there was cautious optimism that the secretive state could call time on its missile programme.
During last month's Singapore summit Kim agreed "to complete denuclearization of the Korean Peninsula", but the document signed by Kim and Trump did not include details of how and when North Korea would denuclearize.
In a report dated July 28, 2017, the U.S. Defense Intelligence Agency estimated that the North possesses as many as 60 nuclear weapons, but Monday's report said that intelligence agencies had in recent months increased their estimates of the size of the arsenal, taking into account enriched uranium from at least one secret enrichment site.
The Post quotes officials as saying that the Sohae Satellite Launching Station on North Korea's west coast is an exception to the "business as usual" assessment.
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Photos and infrared imaging indicate vehicles moving in and out of a facility at Sanumdong, but do not show how advanced any rocket construction might be, a senior U.S. official told Reuters.
Kim Do-gyun, the South's chief negotiator who is in charge of North Korea policy at the defence ministry, told reporters before leaving for the DMZ that he would make efforts to craft "substantive" measures to ease tensions and build trust.
The White House said it did not comment on intelligence, Reuters reported, while a senior official at South Korea's presidential office said US and South Korean intelligence agencies are closely looking into various North Korean movements. In that address, the North Korean leader was unequivocal about what was next, they say.
"I'm afraid that at this point, the United States, the Trump Administration is being taken for a ride", said Sen.
It is possible, the official said, that any new missiles being built may be for further testing of such vehicles and of more accurate guidance systems. "So between this and Kangson, we are seeing an expanded and improved nuclear and missile force". What they're offering is: "They keep the bomb, but they stop talking about it".
That was the case with U.S. negotiations with the Soviet Union during the Cold War, and more recently with Iran, "which continued to build more centrifuges capable of producing nuclear material even as it negotiated with the United States to limit those capabilities", Mr Wit said.