Russia has warned that North Korea is apparently conducting work on a nuclear reactor, saying the ageing facility is in such a "nightmarish state" it could cause a disaster.

US analysts say satellite imagery suggests Pyongyang has restarted a research reactor capable of producing plutonium for weapons at its Yongbyon nuclear complex.

The Interfax news agency has quoted a diplomatic source as saying Russia does not have definite information the work has restarted, but warning of dire consequences should that happen.

"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 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 US-Korea Institute at the Johns Hopkins School of Advanced International Studies says a satellite image from August 31 shows white steam rising from a building near the hall that houses the plutonium production reactor's steam turbines and electric generators.

"The white coloration and volume are consistent with steam being vented because the electrical generating system is about to come online, indicating that the reactor is in or nearing operation," the Washington-based institute said.

It says the reactor can produce six kilograms of plutonium a year.

The Russian source has cautioned the steam could "simply be testing of the generator".

However, a US official who spoke on condition of anonymity believes the reactor has been restarted, saying the amount of steam suggests it was being tested.

He says the North's aim may be to demonstrate that it will not abandon its nuclear programs.

The chief of the United Nations nuclear watchdog, the International Atomic Energy Agency, says the body is following the reports but does not yet have a "clear understanding" of the situation at Yongbyon.

"As we don't have inspectors there, we don't have anything for sure," Yukiya Amano said, declining to comment further.

Nuclear tests in North Korea

North Korea has conducted three nuclear tests since 2006.

In April, Pyongyang announced it would revive the aged Yongbyon research reactor but stressed it was seeking a deterrent capacity.

At that time nuclear experts said it would probably take about six months to get it up and running if it had not suffered significant damage from neglect.

The Yongbyon reactor has been technically out of operation for years.

In 2008 the North destroyed its cooling tower as a confidence-building step in six-nation talks which involved it, South Korea, China, the United States, Japan and Russia

In July North Korea said it would not give up its nuclear deterrent until Washington ended its "hostile policy" towards it, although it was ready to revive nuclear talks.

The country tested a nuclear weapon in February and spent the first three months of 2013 issuing bellicose threats including a warning that it would launch a nuclear attack against the United States and South Korea.

Recently, however, Pyongyang has been on what analysts describe as one of its periodic charm offensives, agreeing with Seoul on Wednesday to re-open a shuttered industrial park on a trial basis.

The Kaesong industrial zone lies inside North Korea and was closed when Pyongyang pulled its 53,000 workers out in April amid rising tensions.

AFP/Reuters