North Korea has warned it could carry out a nuclear test if demands for aid are not met, US officials said.
Washington wants the North's nuclear facilities dismantled
The threat was allegedly made on Thursday on the sidelines of six-nation talks between the US, the two Koreas, China, Russia and Japan.
Unnamed US officials said Pyongyang had made similar threats before, and that the talks had still been useful.
A nuclear test by North Korea would add to regional unease and pressure its neighbours to join a nuclear arms race.
The warning about a possible nuclear test came during one-to-one talks between James Kelly, the US negotiator, and his North Korean counterpart Kim Kye-gwan.
South Korean officials played down the incident, saying the warning had not been a "direct threat".
BREAKING THE DEADLOCK?
US reportedly ready to agree to fuel aid and 'provisional guarantee' not to attack
Talks on lifting US sanctions also on offer
In return, North must seal nuclear facilities within 3 months
Fuel aid and talks will continue if North then dismantles facilities
Correspondents said the North Korean delegation may have raised the issue to stress the pressures it was under from hard-liners in Pyongyang.
North Korea has reportedly demanded massive energy aid in exchange for freezing its main nuclear facility at Yongbyon.
According to Japan's Kyodo news agency, the impoverished state has asked for 2m kilowatts of energy aid, equivalent to that which a previous, now defunct agreement
with the US would have provided.
The sides are thought to have been discussing a US proposal to allow other countries to supply energy aid if North Korea agreed to freeze and then dismantle its nuclear facilities.
The BBC's Charles Scanlon, in Beijing, says that after a year of intermittent talks, the US has significantly changed its tactics.
In the past it said it would not reward North Korea for meeting its international obligations. But pressure from the region, particularly South Korea and China, has persuaded Washington to change its stance.
US officials earlier told The New York Times that the US' allies in the region would send tens of thousands of tonnes of heavy fuel oil to the energy-starved state in return for a commitment from North Korea's leader, Kim Jong-il, to dismantle his plutonium and uranium weapons programmes.
The North would then have three months to seal its nuclear facilities, and the continuation of the oil aid would depend on the dismantlement of the nuclear infrastructure, as verified by international inspectors, the New York Times report said.
It was the first detailed proposal from Washington since President George W Bush took office.
Our correspondent says the US offer seems designed to see if North Korea is serious about a negotiated solution, and to show the region that the US is prepared to be more flexible.
Japan has backed the US offer, while urging North Korea to make clear whether or not it genuinely wants to abandon its nuclear programme.
But Pyongyang has issued no formal response to the proposal.
It appears to have delayed the start of the third day of talks, on Friday, to hold consultations with its closest ally, China.
However, US officials remained upbeat, saying that despite the threat the bulk of the meeting was fruitful.
"The threat isn't anything new and came in the context of long and substantive discussion of our proposal and we left the meeting feeling that they would give the plan careful and serious consideration," one official told the AFP news agency.
The talks are due to end on Saturday.