SEOUL (REUTERS) - North Korea may be preparing to test-launch a new, upgraded prototype of an intercontinental ballistic missile (ICBM), South Korean media reported on Thursday (Jan 19), citing military sources.

In his New Year's speech, North Korean leader Kim Jong Un said the country was close to test launching an ICBM, and state media has said a launch could come at any time. Experts on the isolated nuclear capable country's missile programme believe the claims to be credible.

That test launch could be imminent, and potentially coincide with the inauguration of US President-elect Donald Trump on Friday, South Korean media said.

South Korean intelligence agencies reported on Wednesday that they had recently spotted missile parts being transported, believed to be the lower-half of an ICBM, raising fears that a test-launch may be imminent, the Chosun Ilbo newspaper said, citing unidentified military sources.

"It was different from a conventional Musudan missile in its length and shape," the source told the Chosun Ilbo, referring to the Musudan intermediate-range missile tested by North Korea last year. "It is possible they were moving it somewhere for assembly,"the source said.

North Korea has in the past paraded mockups of a road-mobile missile believed to be an ICBM design which has been dubbed KN-08 by outside observers. It is also believed to have an upgraded version of the missile, known as the KN-14.

A road-mobile ICBM, which could be kept hidden or moving until it was fired, would make the job of tracking and stopping a North Korean missile launch significantly more difficult.

The suspected ICBM spotted by South Korea is made up of two parts under 15m long and is shorter than the KN-08 and KN-14, the Yonhap News Agency said, also citing unidentified military sources.

Last year, North Korea conducted a test of an ICBM engine made up of a cluster of smaller rockets, indicating it was working on an ICBM design.

Separately, the Washington-based think tank 38 North said on Thursday that operations at North Korea's Yongbyon nuclear facility may have restarted. North Korea is believed to be able to reprocess plutonium at Yongbyon used in its nuclear warheads.

38 North said a lack of snow on the roof of the facility visible in satellite imagery indicated the building was being heated and therefore under operation.