North Korea has reversed recent steps it took to restart the Yongbyon plant, the main site of its controversial nuclear programme, US officials say.
State department spokesman Sean McCormack said Pyongyang had put back seals on its nuclear equipment and reinstalled surveillance devices.
The move comes days after the US removed North Korea from its list of countries sponsoring terrorism.
The blacklisting had led to deadlock over the country's nuclear disarmament.
"All the seals are back on, the surveillance equipment is back, reinstalled, and the equipment that had been removed is back where it had been," Mr McCormack said.
"In addition to that, they have removed more rods from the reactor."
But the spokesman said Pyongyang still had work to do on reprocessing and fuel fabrication.
North Korea tested an atomic bomb in 2006, but six-nation talks between North and South Korea, the US, China, Japan and Russia led to an agreement for it to halt all nuclear activity.
The current dispute flared when the North barred inspectors from the International Atomic Energy Agency (IAEA) because it was still on the US list.
The US announced it was removing Pyongyang from the list last Saturday, after the two countries agreed on a series of measures to verify the North's nuclear programme.
The move was hailed by South Korea but criticised by Japan, which is seeking more information about Japanese citizens abducted by the North in the 1970s and 80s.
North Korea allowed the IAEA monitors back to Yongbyon on Tuesday.