Russia warned yesterday 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.
China called for joint efforts to denuclearise the Korean peninsula and the US envoy on North Korea said the reported restart of the reactor would be "a misstep on the part of North Korea".
"To achieve denuclearisation and to maintain peace and stability is what China has been advocating," Chinese Foreign Ministry spokesman Hong Lei said at a daily briefing. "It takes efforts from all sides."
The reactor, completed in 1986, is outdated and North Korea could suffer a major disaster if it is restarted, a Russian diplomatic source told the Interfax news agency.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 on 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 diplomatic source said.
"For the Korean peninsula this could entail terrible consequences, if not a man-made catastrophe."
Speaking in Tokyo, the US envoy Glyn Davies said: "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."
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 picked up by satellites "could simply be testing of the generator," the Russian diplomatic source cited by Interfax cautioned.
The International Atomic Energy Agency said it was unable to verify whether the reactor had been restarted because North Korea has barred its inspectors since 2009.
The restart of the plutonium reactor would undermine years of efforts by the international community to stall and roll back North Korea's pursuit of an advanced nuclear deterrent. It would also call into question the effectiveness of the current policy of non-engagement with Pyongyang.
North Korea declared in April 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 over North Korea, which defiantly carried out a third nuclear test in February and threatened to attack the US over its reaction.
Yet Pyongyang has more recently embarked on something of a charm offensive, agreeing to reopen the Kaesong 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 North Korea build the first nuclear complex at Yongbyon in the 1950s and 1960s, although North Korea itself built the five-megawatt plutonium reactor at Yongbyon, 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 Yongbyon reactor is capable of producing 6kg of plutonium a year - enough for one nuclear bomb.
"They really are putting themselves in a position to increase the amount of material they have for nuclear weapons," said Lewis, of the US-Korea Institute.
"This gives them a little bit of leverage in negotiations, and adds a sense of urgency on our part," he told the BBC.