The top US envoy on North Korea dismissed hopes for an immediate diplomatic solution over Pyongyang's expected nuclear test, saying Monday that the isolated state was "bent on playing a game of risk".

The comments from Glyn Davies come as North Korea threatens daily that it is preparing its third nuclear test in response to UN sanctions imposed on Pyongyang for a long-range rocket launch in December.

Davies, who was in Tokyo to meet with his Japanese counterpart after visiting Seoul and Beijing, said Pyongyang's sabre-rattling showed it was "bent on playing a game of risk", adding: "This is very dangerous."

"I have to be honest with you as a diplomat. I don't see any prospect for a diplomatic process in the immediate (future), as long as North Korea continues this belligerent and provocative behaviour and language," he told reporters in the Japanese capital.

Davies said his Asian trip was intended to "explore a way forward with our partners to a credible and authentic diplomatic process".

"But we found ourselves dealing instead with North Korea bent on bluster and intimidation and a North Korea uninterested in finding a diplomatic way forward."

He added: "We would like to step back from this kind of provocative stance to enable us to get back on a diplomatic process."

New satellite images have revealed ongoing activity at North Korea's atomic test site, with the US-Korea Institute at Johns Hopkins University saying an atomic detonation could happen "in a few weeks or less".

In a meeting with top security officials, North Korean leader Kim Jong-Un ordered "substantial and high-profile important state measures", fuelling speculation of an imminent detonation.

The North's two previous detonations, in 2006 and 2009, followed a similar pattern, of a missile launch, followed by UN measures, followed by a nuclear test.

US warnings about the consequences of a third test have been backed by unusually strident criticism of Pyongyang by its sole major ally China.