The sticking point is North Korea's refusal to allow outside inspectors to take samples at its Yongbyon complex for verification of its declared nuclear activities.

The material "covered not just plutonium, but HEU [highly-enriched uranium] and proliferation activity," McCormack said.

No date has been set for a new round of negotiations.

Different strategy

The move came as the US warned that it will "rethink" its approach to North Korean disarmament after the latest round of six-nation talks collapsed after four days in Beijing.

"There was an open door, and all they had to do was walk through it"

Dana Perino, White House press secretary

Negotiators were unable to bridge a dispute over getting a protocol, in writing, to verify North Korea's account of its past nuclear activities.

"What's unfortunate is that the North Koreans had an opportunity here," Dana Perino, the White House press secretary, said.

"There was an open door, and all they had to do was walk through it."

The negotiations have suffered countless setbacks since they began in 2003 and did not stop Pyongyang from testing its first atomic bomb in 2006.

The US had announced plans to drop North Korea from its so-called terrorism blacklist after reaching an apparent deal on earlier nuclear inspection demands.

That is part of the six-nation pact in 2007 promising North Korea diplomatic concessions and economic incentives, including energy aid, in exchange for total disarmament.

Standoff

North Korea has refused samples to be taken from its Yongbyon nuclear plant [AP]
Earlier Christopher Hill, the chief US envoy to the disarmament talks, blamed Pyongyang for the standoff after it rejected a verification plan for the declaration it made in June.

The six countries in the negotiations – North and South Korea, China, Japan, Russia and the US – are seeking to formalise a verification mechanism agreed to in principle, verbally, in October.

The state department also appeared to suggest that deliveries might be withheld.

"We have yet to see action on the verification protocol from North Korea, so I suspect that ... anything that we might do would be pending North Korea taking the steps that they have obliged themselves to make," McCormack said.

Under a landmark deal in 2007, North Korea was to receive one million tonnes of fuel oil or equivalent energy aid from the other five countries in return for disabling and dismantling its main plutonium-producing plants in Yongbyon.