Under a February 13 multilateral deal, Pyongyang agreed to shut down the Yongbyon plant within 60 days, but it insisted it would not make good on that pledge unless funds, frozen in Macau's Banco Delta Asia after the US accused the bank of being involved in money laundering, were released.
Despite missing Saturday's deadline, Pyongyang said it remained committed to carrying out the February 13 agreement "and will also move" when the money in the Macau bank was released.
The US has said the funds have now been unblocked and should no longer be an issue.
US ultimatum
North Korea said on Friday it would soon check whether it could access the funds, which account for about $25m.
"It is time now for the DPRK to make its move so that all of us can move forward," the US state department said in a written statement, referring to the country by its official name, the Democratic People's Republic of Korea.
The statement also said North Korea must immediately invite back International Atomic Energy Agency nuclear inspectors and begin shutting down Yongbyon to get energy aid promised under the February 13 deal.
Sean McCormack, the state department spokesman, said in the statement: "It remains for the DPRK Korea to realise fully its commitments ... by inviting back the IAEA immediately to begin shutting and sealing the Yongbyon nuclear facility."
McCormack said that would allow other nations to provide North Korea with the equivalent of 50,000 tonnes of heavy fuel oil, as promised under the agreement among the two Koreas, China, Japan, Russia and the US.