Russia said on Thursday that North Korea was apparently conducting work on a nuclear reactor, warning that the ageing facility was in such a "nightmarish state" it could cause a disaster.

"It is obvious that some works are being conducted, and for a long time at that. According to some signs, steps were indeed being taken to relaunch it," the Interfax news agency quoted a diplomatic source as saying.

The source said that Russia did not have definite information that Pyongyang had restarted the plutonium reactor at Yongbyon, as US analysts have suggested based on satellite imagery, but warned of dire consequences if this happened.

"We do not have any information that the reactor has been relaunched," the source said.

"Our main concern is linked to a very likely man-made disaster as a consequence. The reactor is in a nightmarish state, it is a design dating back to the 1950s."

"For the Korean peninsula this could entail terrible consequences, if not a man-made catastrophe."

The white steam picked up by satellites rising from a building next to the reactor "could simply be testing of the generator," the diplomatic source cautioned, however.

In a separate dispatch, Interfax cited a diplomatic source as saying there was little hope for constructive talks on North Korea's nuclear programme.

"The North Koreans are saying 'We are ready to renew talks,' but it's not clear what about," the source said.

"So far the situation is complicated."