The UN nuclear watchdog agency says North Korea has moved fresh fuel to a reactor which the United States says must stay closed due to its capacity to produce plutonium for nuclear weapons.
The announcement by the International Atomic Energy Agency (IAEA) heightens a tense international confrontation that has followed the breakdown of an eight-year-old agreement restricting North Korea's nuclear programme.
North Korea's defence minister accused Washington on Tuesday of pushing the Korean peninsula to the brink of nuclear war.
"We had noticed yesterday that they were carrying out work at the five megawatt reactor in Yongbyon," IAEA spokesman Mark Gwozdecky told journalists. "And we noticed that they were moving fresh fuel to the reactor."
He added that North Korean technicians had broken most of the seals and disabled U.N. surveillance devices at all four nuclear facilities at Yongbyon. The cameras had been monitoring North Korea's compliance with a 1994 shutdown of the plants.
"North Korea estimates that (the five megawatt reactor) could be up and running in one to two months," he said, adding that the U.N. agency believed it would take longer.
The IAEA is also worried about the plutonium storage and reprocessing facilities at the Yongbyon complex. A storage pond there holds some 8,000 spent irradiated fuel rods which contain large amounts of plutonium.
The facilities at Yongbyon were closed under a 1994 agreement with the United States under which North Korea halted its nuclear arms programme in exchange for oil shipments and the construction of two atomic reactors that are difficult to use for military purposes.
The United States suspended oil shipments to North Korea this month after revelations in October that it was operating a separate nuclear weapons programme using highly enriched uranium.
On Saturday, North Koreans began removing the seals and disabling U.N. monitoring cameras at the five-megawatt Yongbyon reactor after the IAEA failed to meet Pyongyang's demand that it take away the gear so it could revive the reactor.
Gwozdecky said the IAEA was keeping two inspectors in North Korea to keep on eye on the situation. It has carried out limited inspections of North Korea's nuclear facilities since the early 1990s.
U.S. intelligence officials say enough weapons-grade plutonium had already been produced at Yongbyon to build two nuclear weapons by the time the plant was closed down in 1994.