A sticking point in talks has been how to verify North Korea's disarmament
North Korea has asked the UN nuclear watchdog to remove its seals from a key atomic facility, the organisation says.
Pyongyang said earlier it would restart the Yongbyon plant amid disputes over a disarmament-for-aid deal.
North Korea said Washington had not taken it off a list of state sponsors of terrorism, as promised.
US President George W Bush "expressed his concern" to China's Hu Jintao and the leaders agreed to work to convince Pyongyang to resume denuclearisation.
"The two presidents agreed that they would work hard to convince the North to continue down the path established in the six-party talks," a White House spokesman said.
Pyongyang was expecting to be removed from the US terror list after submitting a long-delayed account of its nuclear facilities to the six-party talks in June, in accordance with the disarmament deal it signed in 2007.
It also blew up the main cooling tower at Yongbyon in a symbolic gesture of its commitment to the process.
But the North now says it no longer wants US incentives and will push ahead with plans to resume operations at Yongbyon.
The removal of similar seals in December 2002 sparked a long-running crisis which resulted in Pyongyang testing an atomic weapon in 2006.
The director of the International Atomic Energy Agency (IAEA), Mohammed El Baradei, told reporters that North Korea asked inspectors to remove the agency's seals and surveillance cameras on Monday morning.
Mr ElBaradei said this was "to enable them to carry out tests at the reprocessing plant, which they say will not involve nuclear material".
A senior diplomat close to the IAEA told Reuters that the seals had been removed.
Mr ElBaradei also said that inspectors at Yongbyon had observed that some equipment previously removed by North Korea during the disablement process had been brought back.
However, this did not "change the shutdown status of the nuclear facilities at Yongbyon", he added.
Last week, North Korea claimed that the process of decommissioning the plutonium-producing reactor at the Yongbyon plant was 90% complete.
But they said the process had been halted in response to the US delay in removing it from a list of countries it says supports terrorism.
Foreign ministry spokesman Hyun Hak-bong said North Korea was "proceeding with works to restore [the reactor] to its original status".
Mr ElBaradei said he hoped that the conditions could be created for North Korea to return to the nuclear non-proliferation treaty at the earliest possible date.
Experts believe Yongbyon would take a year to restore, a view supported by a recent IAEA report.