North Korea says it has restarted, or is set to restart, the atomic plant at the heart of its suspected nuclear arms programme.
The United States called the move a dangerous development by a "terrorist regime".
An English-language North Korean Foreign Ministry statement late last night apparently said the reactor had been started to generate electricity for the energy-starved communist country.
"The DPRK [North Korea] is now putting the operation of its nuclear facilities for the production of electricity on a normal footing after their restart," the statement said.
An South Korean official said the statement was less clear in Korean and could be taken to mean "poised to restart".
Nuclear experts say the Yongbyon plant is too small to produce much power but could produce material for atomic bombs.
"The situation in North Korea is a dangerous one," US Defense Secretary Donald Rumsfeld said in Washington. "It's a regime that is a terrorist regime . . . and the fact that they have announced that they are going to breach three or four agreements . . . is a worrisome thing".