The US has urged North Korea to fulfil its pledge to shut down the Yongbyon nuclear reactor, as Pyongyang missed a key deadline for closing the plant.
North Korea has missed a deadline to shut down its main reactor
North Korea should "immediately" invite nuclear experts to begin sealing the facility, the US state department said.
But the US said it remained committed to the landmark 13 February agreement.
Under the deal, North Korea agreed to close Yongbyon within 60 days in return for aid. But a row over funds frozen in Macau has stalled progress on the deal.
US State Department spokesman Sean McCormack said it was time for North Korea "to make its move so that all of us can move forward".
In Beijing, US nuclear negotiator Christopher Hill said that North Korea's failure to meet the 14 April deadline was cause for concern.
But he said that Chinese officials had urged the US to show patience and to wait "a couple more days" for Pyongyang to comply.
"They said the lines of communication were open... if it is going to get resolved it certainly should be able to get resolved very soon," he said.
The implementation of the deal - aimed at ending North Korea's nuclear programme - has been delayed by a row over $25m (£13m) of North Korean money held in a Macau bank.
N KOREA NUCLEAR DEAL
N Korea to 'shut down and seal' Yongbyon reactor, then disable all nuclear facilities
In return, will be given 1m tonnes of heavy fuel oil
N Korea to invite IAEA back to monitor deal
Under earlier 2005 deal, N Korea agreed to end nuclear programme and return to non-proliferation treaty
N Korea's demand for light water reactor to be discussed at "appropriate time"
In 2005 the US accused Banco Delta Asia (BDA) of acting as a conduit for money earned by Pyongyang from illegal activities, and the accounts were frozen.
After the February deal, the US unblocked the funds but finding a way to transfer the money took several days. However, the US now says the dispute has been resolved.
North Korea - which insists that progress on implementing the deal is contingent on getting the money back - said on Friday that it would "confirm soon" whether the funds had been released.
Mr McCormack said that if North Korea allowed International Atomic Energy Agency experts in to begin sealing Yongbyon, other countries could then move forward with the provision of heavy fuel oil as agreed under the deal.
A senior US official told journalists that the US was prepared to give North Korea more time.
"Our patience is not infinite... but we feel that given that the kind of unexpected complexities that did arise in connection with some of the banking issues that it's probably prudent to give this thing a few more days," Reuters news agency quoted the official as saying.
On Friday, North Korean foreign ministry official Kim Son-gyong told AFP news agency that Pyongyang would "respect our commitment" to the deal.
"There is no reason to be pessimistic. We will be faithful to this agreement if the Americans respect its clauses," he said.
The deal followed heightened tensions on the Korean Peninsula after North Korea test fired a new long-range missile in July and then held its first nuclear test in October.