A senior US official warned North Korea Monday against conducting another nuclear test, saying it would be a "serious mistake" that would incur more sanctions and further isolation.

"I think it will be a serious miscalculation and mistake if North Korea were to engage in a nuclear test," Glyn Davies, US special envoy for North Korea policy, told reporters.

He was speaking after a meeting in Seoul with his South Korean and Japanese counterparts, Lim Sung-Nam and Shinsuke Sugiyama, to discuss the aftermath of the North's long-range rocket launch on April 13 and other issues.

Expectations of a third atomic test by Pyongyang have grown since the United Nations Security Council condemned the North's failed launch and tightened existing sanctions.

The North said its aim was only to put a peaceful satellite into orbit but the UN said it breached a ban on testing ballistic missile technology.

North Korea responded with nuclear tests after the UN imposed sanctions against its rocket launches in 2006 and 2009.

Satellite photos last month showed work under way at its atomic test site but gave no indication of whether or when a test would be staged.

"This new regime in Pyongyang saw that the world community... was united in reacting to the missile launch," Davies said, warning of "swift and assured reactions" by the UN to any nuclear test.

Group of Eight leaders expressed "deep concerns" Saturday at the North's "provocative actions" and condemned the rocket launch.

"We affirm our will to call on the UN Security Council to take action, in response to additional DPRK (North Korean) acts, including ballistic missile launches and nuclear tests," they said in a statement after a summit.

Davies will leave for Beijing on Tuesday, where he plans to meet China's chief nuclear envoy Wu Dawei, and will travel on to Tokyo Wednesday.

A US-North Korean deal announced on February 29 had raised hopes of progress under Pyongyang's new leader Kim Jong-Un.

The North had pledged to suspend uranium enrichment and impose a moratorium on nuclear and missile tests in return for 240,000 tonnes of US food aid.

Pyongyang insisted its launch was not a missile test but Washington suspended the food aid, saying the North could not be trusted.

"They sent a signal that they can't be trusted to follow through on their own undertakings and their own promises," Davies said.

"Words are no longer, quite frankly, interesting to us. What we want to see is actions from North Korea."