The North staged a nuclear test in October 2006 but later agreed to return to six-party negotiations grouping the two Koreas, the United States, China, Russia and Japan.

A landmark deal reached on February 13, 2007 offers the North a million tons of fuel oil or equivalent energy aid, normalised relations with the United States and Japan and a formal peace treaty, if it scraps all nuclear programmes and material.

In the current phase the North agreed to disable its atomic plants and fully declare all nuclear programmes by the end of last year. But it missed the deadline amid a dispute with the US over the declaration.

Pyongyang says it submitted a list in November. But the US says it must account fully for a suspected secret UEP (uranium enrichment programme), which the North has consistently denied having.

"It is undoubtedly North Korea that should be responsible for presenting evidence to clarify the UEP," Mr Chun said. "It is a daunting task but not an issue that is impossible to settle."

The South Korean envoy did not expect the declaration issue to derail the six-nation process, but reiterated a call for the North to give a 'complete and correct' declaration to keep the process afloat.

The envoy warned against any prolonged deadlock. "We will not be able to let the current situation go on indefinitely like this," Chun said.

North Korean official media say Pyongyang has slowed down the disarmament work in response to what it sees as the failure of negotiating partners to keep their side of the agreement.

In particular, it wants the US to start the process of removing it from a list of state sponsors of terrorism. Washington says this depends on receiving a full declaration.

Asked if the North was really willing to give up all its nuclear material, Mr Chun said it was premature to judge. "We have to wait and see."