The US has urged China to help rein in North Korea’s nuclear programme after reports Pyongyang has built an extensive new uranium enrichment facility that could add to the country’s stock of atomic weapons material.

President Barack Obama has sent Stephen Bosworth, his special envoy for North Korea, to Seoul, Tokyo and Beijing in the wake of Pyongyang’s claim it has built a facility with two thousand centrifuges to enrich uranium. The process can yield both nuclear fuel and fissile material for weapons. The response from South Korean atomic negotiators was muted. They said they had no idea how far Pyongyang’s nuclear scientists had got or where experiments might be running.

Admiral Mike Mullen, chairman of the joint chiefs of staff, said: “We have to continue to bring pressure on [Kim Jong-Il].” He described North Korea as a “very dangerous country” led by an unpredictable leader who had once again managed to destabilise the region. “A great part of this, I think, will have to be done through Beijing,” he added.