The communist North denies the existence of such a programme, the issue that triggered the crisis 20 months ago and led to three rounds of inconclusive six-nation talks in Beijing.

The third round closed with a bland agreement to meet again before the end of September and a pledge to take the first steps to resolve the crisis "as soon as possible." Working-level talks would be held in late July, Russia's envoy to the talks said.

China's chief negotiator, Wang Yi, said the main gap was between the United States and North Korea.

"There are serious differences between the two sides over the uranium enrichment programme," Wang told a news conference after the talks closed. "We hope that this question, together with other issues, will be clarified and resolved in future talks."

The parties had agreed that a freeze of the North's nuclear activities should be a first step, he said.

North Korea stressed its readiness to freeze plutonium-based nuclear facilities but adamantly refused to accept the US demand that it admit to having a uranium enrichment programme, used for making bombs, a diplomatic source in Beijing said.

North Korea also rejected proposals by the United States and Japan to allow International Atomic Energy Agency (IAEA) experts to inspect its nuclear facilities for verification. The source said Pyongyang had demanded a "different form of inspection."

North Korea pulled out of international agreements on non-proliferation and threw out IAEA inspectors just weeks after the crisis erupted in October 2002, when US officials said Pyongyang had admitted to a clandestine nuclear programme.

It also reactivated its mothballed atomic plant at Yongbyon, north of Pyongyang.

The discussions in Beijing were buoyed at the outset by the first detailed US proposal to end the crisis. It offered Pyongyang security guarantees and South Korean aid in return for North Korea agreeing to fully dismantle its nuclear programmes.