But the US played down Seoul's allegations on Wednesday, saying North Korea had only moved some equipment out of storage from its ageing nuclear plant in Yongbyon.
US officials, who spoke on condition of anonymity, said they viewed North Korea's moves more as a negotiating tactic than genuine effort to rebuild Yongbyon, which proliferation experts believe has produced enough plutonium for six to eight bombs.
A US counter-proliferation official said Washington had seen evidence that some parts had been moved back to their original Yongbyon site but described this as "mostly symbolic, mostly done for effect in response to the negotiations".
"It's a step designed to show that they can take more steps if they want to," said the official. "It doesn't mean that it's not real. It doesn't mean that it's not dangerous. But people shouldn't read too much into it," the official said.
Last week North Korea said it stopped dismantling its nuclear reactor in Yongbyon on August 14 and threatened to restore the nuclear facility, citing Washington's failure to keep its promise to remove Pyongyang from its blacklist of state-sponsors of terrorism.
The US state department, citing information from US nuclear experts still working at Yongbyon, said the North had not begun reconstructing the facility.
"Our understanding is that the North Koreans are moving some equipment around that they had previously put into storage," Sean McCormack, a department spokesman, said.
"Based on what we know from the reports on the ground, you don't have an effort to reconstruct, reintegrate this equipment back into the facility."
Reports in the South Korean media quoted a senior foreign ministry official as saying that nuclear envoys from South Korea and the US are expected to meet in Beijing on Thursday to discuss the North's moves.
But the foreign ministry would not confirm the report.
In June, North Korea destroyed the cooling tower of the Yongbyon facility in a public demolition to demonstrate its commitment to scrapping its nuclear programme.
It began disabling the plutonium-producing facilities in November, but slowed the work over the dispute on how to verify the nuclear declaration it had handed over to the US.
In a new report, the UN's nuclear watchdog said North Korea had already removed "essential" equipment from its nuclear plant by the time it decided to stop disabling them last month.
South Korean and US officials have said that it would take at least a year for North Korea to restart the facilities after they are completely disabled.