SEOUL (AFP) - North Korea could carry out another nuclear test at any time, a US think tank has suggested, after analysing satellite imagery from Pyongyang's main testing site.

Activity at the Punggye-ri underground facility suggested the North was maintaining tunnels as well as cleaning up after its internationally condemned detonation in January, the US-Korea Institute at Johns Hopkins University said on Friday (March 18).

"It is highly likely that (the) site is capable of supporting additional tests at any time," it said on its closely watched 38North website.

Satellite images taken this month supported theories that there were unused test chambers at the site and showed activity around the main support area, with numerous vehicle tracks and footpaths visible, it said.

At one entrance to the test tunnels, snow had been cleared, indicating that they were at the very least being maintained for future tests, the think tank said.

North Korean leader Kim Jong Un has ordered an imminent nuclear warhead explosion test and multiple ballistic missile launches, Pyongyang's state media said this week, ratcheting up Pyongyang's face-off with the international community just days after being slapped with tough UN sanctions.

The North carried out its fourth nuclear test on Jan 6, followed a month later by a long-range rocket launch, apparently a disguised ballistic missile test.