North Korea's chief nuclear envoy appealed for long-stalled talks on its atomic programme to resume "without preconditions" on Thursday, Korean media said, days after reports it may have restarted a reactor.

The comments in Beijing by Pyongyang's First Vice Foreign Minister Kim Kye-Gwan were the North's following suggestions -- based on satellite imagery -- that the main plutonium reactor at its ageing Yongbyon nuclear facility may be back up and running.

The development last week was met with grave concern and Russian warnings of a Chernobyl-like disaster given the reactor's "nightmarish state".

The US added that restarting the facility, which was built with Soviet help and mothballed in 2007 would be "a misstep" and a "very serious matter".

The United States and South Korea have long demanded that Pyongyang show commitment to ending its nuclear weapons programme before the six-party talks, which have been stalled for several years, can resume.

But according to the Yonhap News Agency, Kim told the meeting in Beijing: "Attaching preconditions to our offer of dialogue would cause mistrust."

China is the North's sole major ally and economic lifeline, and Kim told the forum, marking the 10th anniversary of the negotiations' launch: "We are ready to enter the six-party talks without preconditions."

The North has said for years it wants denuclearisation of the whole Korean peninsula and that it is developing a nuclear arsenal to protect itself from the US military, which occasionally sends nuclear-powered warships and aircraft capable of carrying atomic weapons to the South for drills.

"Denuclearising the Korean Peninsula is a dying wish of our late leaders and our country's policy goal," Kim said.

China regularly also calls for the denuclearisation of the Korean peninsula, and has tended to prioritise regional stability in its relations with the North.

South Korea, which does not have nuclear weapons, and ally the United States, have made it clear they will never accept the idea of North Korea as a nuclear state.

They insist Pyongyang must show a tangible commitment to abandoning its atomic arms if it wants substantive talks.

In February the North carried out its third underground nuclear test in defiance of UN Security Council resolutions, sending tensions soaring and raising fears of possible conflict. It also launched a rocket in December that Washington said was a disguised ballistic missile test.

As well as the two Koreas, China and the US the six-party talks also involve Russia and Japan.