Russia on Thursday warned of a potential "man-made catastrophe" if North Korea restarts an ageing plutonium reactor to boost its stockpile of nuclear weapons, after US experts spotted steam rising from the Yongbyon facility.

The reactor, which was completed in 1986, is outdated and North Korea could suffer a major disaster if it is restarted, a Russian diplomatic source told the Interfax news agency.

The warning came after researchers at the US-Korea Institute said Wednesday that satellite images taken on August 31 showed plumes of white steam rising from a building next to the reactor.

"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," the Russian source said.

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

The US envoy on North Korea meanwhile said the reported restart of the reactor would be "a misstep on the part of North Korea".

"If it turns out that these reports are true that North Korea has restarted the five-megawatt plutonium reactor, this would be a very serious matter," Glyn Davies told reporters after meeting Japanese foreign ministry officials in Tokyo.

Such a move would "seriously violate the United Nations Security Council resolutions" and North Korea's commitments under a 2005 joint statement after six-party talks, Davies said.

The Russian diplomat speaking to Interfax said he did not know for sure whether North Korea had relaunched the facility mothballed in 2007.

"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 diplomat said.

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

The image examined by researchers at the US-Korea Institute shows that North Korea "appears to have put the reactor into operation", researchers Nick Hansen and Jeffrey Lewis wrote on the institute's blog, 38 North.

But the white steam "could simply be testing of the generator", the Russian diplomatic source cited by Interfax cautioned.

The Vienna-based International Atomic Energy Agency said it was unable to verify the claim since North Korea has barred its inspectors since 2009.

"We are aware of the media report," said spokeswoman Gill Tudor.

"The agency continues its monitoring of the DPRK's nuclear activities by available means, such as satellite imagery analysis."

North Korea declared in April that it would restart all facilities at Yongbyon to "bolster the nuclear armed force both in quality and quantity".

The pledge came at a period of high international tension after North Korea defiantly carried out a third nuclear test in February and threatened to attack the United States over its reaction.

Yet Pyongyang has more recently embarked on something of a charm offensive, agreeing to reopen a joint industrial zone with South Korea and to resume reunions of families separated by the Korean War.

The Soviet Union played a key role in helping build the first nuclear complex at Yongbyon in the 1950s and 1960s, although North Korea itself built the plutonium reactor, which became operational in 1986.

In 2007, North Korea shut down the Yongbyon reactor under a six-nation aid-for-disarmament deal and publicly knocked down its cooling tower.

The reactor was the totalitarian state's sole way of producing plutonium, which it used to conduct its first two nuclear tests in 2006 and 2009.

Pyongyang is currently believed to have enough plutonium for about six bombs. The Yongbyon reactor is capable of producing six kilogrammes (13 pounds) of plutonium a year -- enough for one nuclear bomb.

In recent weeks, North Korea has indicated its willingness to resume six-party talks on its nuclear programme, involving the United States, Russia, China, Japan and South Korea.

The US point man on North Korea said Thursday that Washington hopes for "meaningful, authentic and credible six-party talks".

But he also added: "There is an issue right now what are six-party talks to be about," insisting that Pyongyang could not set any agenda beyond scrapping its nuclear programme.

burs-am/gk