US intelligence satellites have picked up signs of activity at North Korea's Chamjin missile factory southwest of Pyongyang, according to two officials familiar with the matter.
The US believes the activity could signal preparations for some type of ballistic missile test in the first days or weeks of the Trump administration, they said.
On Thursday, South Korea's Yonhap news agency quoted unnamed Korean military officials as saying two missiles have been placed on mobile launchers. The devices "are estimated to not exceed 15 meters (50 feet) in length, making them shorter than the North's existing ICBMs."
An official from Seoul's Joint Chiefs of Staff told reporters the claims could not be confirmed but said the military was monitoring the situation closely.
Pentagon spokesman Peter Cook would not comment on the intelligence in his daily briefing.
An ICBM test in the coming days is "highly plausible," Andrei Lankov, a professor at Seoul's Kookmin University, told CNN.
"Judging by earlier behavior they usually like to greet a newly elected American president with some kind of nice surprise like a nuclear (test) or missile launch," he said.
North Korea conducted its second nuclear test
early during Obama's first term, and its third just a month into his second
. Early last year, Pyongyang said it had successfully conducted a hydrogen bomb test
This week, US-based monitoring service 38 North warned of increased activity at the Yongbyon nuclear site
, which may indicate North Korea is attempting to produce more plutonium to fuel future nuclear weapons.
"Because President-elect Trump tweeted that 'it won't happen
,' such a launch could be seen as a serious humiliation for (the US)," Lankov said.
Last week, Tal Inbar, a North Korea expert at the Fisher Institute for Air and Space Strategic Studies, told CNN that North Korea would likely want to test an ICBM before any potential negotiations
with the Trump administration, in order to strengthen Pyongyang's hand.
Any potential ICBM test, while a propaganda win for Pyongyang, would also reveal a great deal about the progress of North Korea's weapons program.
Military experts speaking to Yonhap predicted that any test would only involve a missile with a range of under 2,500 kilometers (1,550 miles), less than half the usual ICBM range of 5,500 km (3,400 miles).
North Korea has never successfully tested an ICBM, despite repeatedly showing off what it claims are working missiles at military parades. Some experts have publicly doubted
whether these missiles are anything but mock-ups.
While Inbar disagrees with that assessment, he said that repeated testing will have to take place
before Pyongyang could be confident of a missile program capable of actually hitting a strategic target.
Lankov pointed out that previous weapons tests have gone "seriously wrong," but added that North Korea "has demonstrated remarkable, almost unbelievable progress in developing nuclear and missile systems (in recent years)."
It took only a handful of years "for them to develop from scratch and successfully test launch a (submarine-launched ballistic missile
), most analysts thought it would take a decade, maybe more," he said.