The US has said it has no confirmation that North Korea is starting to shut down its main nuclear reactor.
North Korea has missed a deadline to shut down its main reactor
Comments by State Department spokesman Sean McCormack follow media reports in South Korea that Pyongyang may have begun dismantling the Yongbyon reactor.
Satellite images detected unusual activity there, the Dong-a Ilbo newspaper and Yonhap news agency said.
The media reports raised hope that North Korea still intended to comply with an international deal.
Pyongyang has already missed the first deadline agreed as part of the deal.
It was meant to have "shut down and sealed" its Yongbyon reactor by Saturday - 60 days after the deal was reached.
But complications over North Korean funds, frozen in a Macau bank, delayed proceedings.
On Tuesday, Mr McCormack said Washington was willing to give North Korea "a little bit more time" to honour its pledge.
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"
Pyongyang linked progress on the agreement to the return of the money, refusing to move forward with shutting down the reactor until it had access to the $25m (£13m).
After weeks of delay, the US said recently that the money was now available to be picked up, but there has been no confirmation of this from Pyongyang, and the funds are currently still in Macau's Banco Delta Asia.
Saturday's deadline came and went with no official comment from the North.
Since the deadline has passed, there has been huge pressure on the North to start the process of shutting down its Yongbyon reactor as soon as possible.
South Korean Foreign Minister Song Min-soon spoke by telephone with his US counterpart, Condoleezza Rice, on Tuesday and the two "strongly expressed expectations that North Korea will soon implement disarmament measures," the South's Foreign Ministry said in a statement.
South Korean media has also reported that Seoul is thinking of delaying the resumption of rice aid to its impoverished neighbour, until the North takes concrete steps forward.
The two Koreas began talks on Wednesday to discuss the North's request for 400,000 tonnes of rice.
But there are signs of hope, according to reports in Tuesday's South Korean media.
Officials from both Seoul and Washington have been closely monitoring activity at Yongbyon - and they have begun to notice changes.
South Korea has reportedly threatened to delay aid shipments
An unnamed intelligence official told Yonhap that the reactor was still operating, but there were some more intense signs of activity than usual.
"We are tracing and analysing them," the official said, requesting anonymity.
Some diplomatic officials were noncommittal about what these signs meant, but another un-named source is quoted as telling the Dong-A Ilbo: "Washington thinks it is highly likely that those activities are a part of North Korea's operations to close down the nuclear facility."
Meanwhile the Macau bank involved in the funding dispute has challenged a US Treasury ruling that bars US banks from doing business with it.
Banco Delta Asia said the ruling ignored all remedial measures it has taken, and "was politically motivated because it was based on disputes between the United States and North Korea".
Last month, the US blacklisted the bank, calling it a "willing pawn" in North Korea's illegal activities.