The US has acknowledged that North Korea may miss the mid-April deadline to cease operations at its main nuclear reactor, saying it was open to an extension of the 60-day deadline, recognising that all parties were "working in good faith".
But Washington said it would not accept a partial shutdown of the facility.
Christopher Hill, the chief US nuclear negotiator, said North Korea was having problems meeting the deadline.
But he urged Pyongyang to keep its side of the bargain regardless of a dispute over the transfer of $25m in frozen North Korean funds.
"Clearly we are aiming for a complete fulfilment of the February agreement and we'd like to get it done by day 60," Hill said in Tokyo on Monday.
"But obviously that timeline is becoming difficult. But certainly there is no such thing as partial," he added.
Hill, who met Japanese officials late on Monday, is scheduled to hold more nuclear talks in Seoul and Beijing throughout the week.
Japan, a signatory to the nuclear pact, has insisted that the funds row should not be used as an excuse for the North to renege on its agreement to dismantle its nuclear programme.
Yasuhisa Shiozaki, a top government spokesman, said the fund dispute should not hold up implementation of the February agreement.
"The [funds] issue is outside the framework of the six-party talks. They cannot make that an excuse not to abide by the 30- or the 60-day deadlines. We need to resume the six-party process," he said.
Meanwhile in Washington, Sean McCormack, a US State Department spokesman, said the money issue "was more complicated that anyone could have imagined", suggesting a possible extension in the deadline.
The State Department last week said that the US had found a way for the frozen funds to be transferred to North Korea, but on Monday appeared less certain this may resolve the dispute, saying there may be other ways to settle the matter.
He said it was up to North Korean, Chinese and Macanese authorities to decide how to handle the funds, which were frozen after the US designated the Macau bank holding the North Korean funds a "primary money-laundering concern" in September 2005.
"We'll take a look at where we are on Saturday," he added.
US legislators visiting the North have expressed confidence over Pyongyang's will to disarm and improve ties with their country.
Bill Richardson, a US presidential candidate who is heading the delegation, said: "They know that the key is dismantling their nuclear weapons."
But one of the potential candidates to be South Korea's next president did not share the same optimism.
Park Geun-hye said on Monday that North Korea was becoming more of a threat by dragging out the process of dismantling its nuclear weapons programme.
"Time only serves to turn North Korea's weapon development into a fait accompli," she said.
"It is quite worrying for many Koreans that the February 13 agreement does not explicitly mention the North's existing nuclear weapons and nuclear material."
Güncelleme Tarihi: 20 Eylül 2018, 18:16