The United States says shipments of fuel oil to North Korea are being stopped because of Pyongyang's refusal to accept a plan for verifying its nuclear program. The action comes a day after the breakdown of talks in Beijing on a verification protocol.

State Department officials say the five powers negotiating with North Korea on ending its nuclear program have agreed to halt deliveries of heavy fuel oil to that country unless it accepts a plan to verify its disarmament steps.

The United States, Russia, Japan, China and South Korea have been engaged in the Chinese-sponsored six-party talks with North Korea, which has agreed in principle to scrap its nuclear program, including weapons, in return for aid and diplomatic benefits.

All the participants except Japan have been providing North Korea with industrial-grade fuel oil in return for North Korea's pledge to disable the country's Yongbyon nuclear reactor complex.

At a news briefing, State Department Spokesman Sean McCormack said in the wake of the failed Beijing talks, the other powers reached an understanding to halt fuel shipments in the absence of a verification deal.

"This is an absolute matter of principle," he said. "And we have been very clear on that, that is an action-for-action negotiation. The sine qua non [absolute condition] for progress is a verification protocol. The ball is in the North Koreans' court."

The Beijing meetings foundered over North Korea's refusal to accept a Chinese draft verification plan that called for nuclear inspectors to be able to remove samples from North Korean nuclear sites for outside analysis.

The United States said Pyongyang verbally accepted sampling in talks with U.S. officials in October. North Korea later denied making such a commitment and said sampling would violate its sovereignty.

The other five parties to the talks say the verification protocol is essential to confirm the accuracy of the declaration of its nuclear activities and holdings that North Korea made last June.

A verification plan would clear the way for the concluding phase of the disarmament deal under which Pyongyang would scrap its nuclear program entirely and get broader diplomatic recognition and security guarantees.

Spokesman McCormack said the chief U.S. delegate to the six-party talks Christopher Hill briefed Secretary of State Condoleezza Rice Friday on the failed Beijing round.

He stressed there is still an opportunity for North Korea to sign on to the verification plan and keep the process, including oil shipments, going.

North Korea was promised one million tons of heavy fuel oil for shutting down and disabling the Yongbyon complex. More than half the fuel has already been delivered.