US officials have said North Korea has restarted the reactor at the heart of its suspected drive for nuclear weapons.
The officials said there were no signs North Korea had reactivated its nuclear-fuel reprocessing plant, which would be of even greater concern because it would take the North a step closer to adding to the two nuclear bombs it is believed to have.
"Part of this demonstrates their desire to continue their nuclear weapons programme and it's another effort to apply pressure on the United States," another US official said.
Analysts in Seoul saw the move as yet another North Korean attempt to shake new South Korean President Roh Moo-hyun, who has been at odds with Washington over how to deal with the crisis. The North upstaged Mr Roh's inauguration on Tuesday by firing a short-range missile into the Sea of Japan.
Initial reaction in South Korea was muted, as Mr Roh and his new prime minister put the finishing touches to their cabinet.
But Bush administration officials have seemed increasingly convinced Pyongyang is determined to launch full-scale production of nuclear weapons. US Secretary of State Colin Powell visited Asia this week where he discussed North Korea and Iraq.
North Korea restarting its reactor did not automatically mean it would next start reprocessing nuclear fuel, but such a move would not be surprising, another US official said.
An more significant step would be movement of 8,000 spent fuel rods, that have gone through the reactor, from a holding pond where they have been stored under the 1994 agreement. Plutonium can be extracted by reprocessing the rods