The negotiations in Beijing between the United States, North and South Korea, Japan, Russia and host China entered the third day on Thursday with an impasse over Pyongyang's insistence on its right to a civilian nuclear energy programme.
Officials from the US and North Korea, the two main protagonists in the negotiations, met for about 90 minutes, but no progress was reported.
"We understand they were not able to narrow differences," a South Korean official said.
Failure to reach an accord in Beijing could prompt the US to take the issue to the UN Security Council and press for sanctions. China opposes such a move, and communist North Korea has said sanctions would be tantamount to war.
"The DPRK has been engaged in nuclear energy for some 25 years. They have not succeeded in turning it into electricity. They have succeeded in turning it into plutonium," chief US negotiator Christopher Hill said, referring to Pyongyang's official name, the Democratic People's Republic of Korea.
The US says satellite photos
show reactor work has resumed
Corey Hinderstein, deputy director of the Washington-based Institute for Science and Internationational Security, says new satellite photos show for the first time that North Korea has resumed some work on a nuclear reactor that could enable it to increase stocks of weapons-grade plutonium.
He claims the pictures appear to confirm earlier reports that the North Koreans had unloaded and restocked a smaller plutonium-producing reactor at its Yongbyon nuclear complex.
Asked to comment on US President George Bush's endorsement of Iran's right to civilian nuclear energy, Hill said: "I think the president's words speak for themselves.
"But I think the main issue here with the DPRK is we put together a comprehensive package to deal with the issue here and they've chosen to focus on something that is not in the package."
Hill (L) described the deal being
offered as 'pretty good'
The US, which once described North Korea as part of an "axis of evil" along with Iran and pre-war Iraq, insists that Pyongyang dismantle all nuclear programmes verifiably and irreversibly, after which it could expect energy aid and security guarantees.
The North wants aid and security guarantees first and the right to keep civilian nuclear programmes. It has also demanded a light-water reactor, a nuclear reactor which generates electricity but is unsuitable for making nuclear weapons.
Draft joint statement
Washington has urged North Korea to focus on a draft joint statement, which sets out the principle of a nuclear-free Korean peninsula and contains an offer from South Korea to supply the North with 2000 megawatts of electricity, roughly equivalent to the North's total power output, if it scraps its nuclear plans.
But Seoul has said it would not be opposed in principle to Pyongyang having a civilian atomic energy programme in future.
"We understand they were not able to narrow differences"
South Korean official
"If their concern is electricity, there is a very generous electricity package. If their concern is something else, they ought to be clear with us and tell us what that is," Hill said.
"Light-water reactor is a non-starter. We have a pretty good deal on the table," he added. "Our view on the talks is we are prepared to participate in the talks as long as we believe the talks are in fact useful."
The latest talks resumed on Tuesday, five weeks after a marathon 13-day session at which the six countries failed to reach agreement even on a statement of basic principles.
Negotiations first began in 2003.
The crisis erupted in October 2002 when Washington said Pyongyang had admitted to a secret programme to enrich uranium, used to make nuclear weapons, in violation of a 1994 agreement.
North Korea denied the charge at the time, and responded by throwing out UN weapons inspectors at the end of 2002 and withdrawing from the Non-Proliferation Treaty in January 2003.
Last February, the North said it had nuclear bombs.