Pyongyang insists it can stand up to South without help from China

A day after leaked diplomatic cables suggested that its crucial ally China could be willing to let its government fall, North Korea yesterday wielded the club of nuclear intimidation and boasted for the first time of its rapidly expanding weapons programme.

Wikileaks' massive cache of private US government communications has greatly complicated the political and military calculus on the Korean peninsula, less than a week after Pyongyang reminded the world of its unpredictability with an attack on the South Korean island of Yeonpyeong.

After the revelation that US officials believed Iran had received sophisticated missiles from the Korean government came a further cache of cables that claimed a growing body of opinion within the senior Chinese leadership would not intervene if the reclusive state collapsed. They suggest that China is much less informed about North Korea's strategy than observers had previously thought.

But as questions remained yesterday over how Pyongyang's calculations might change in the light of a public suggestion that its support from China was not absolute, North Korean strategists claimed the country was nearly finished with construction of a reactor that enriches uranium from several thousand centrifuges, the same model as the reactor that Iran has repeatedly said it needs only to produce electrical energy.

The revelation, which follows a tour of the reactor by a team of American scientists earlier this year, came on the third day of massive US-led war games in the Yellow Sea. US and South Korean forces have staged the war games, which end today, to the din of North Korean threats of "all-out war", "merciless punishment" and "unforeseen consequences".

Rodong Sinmun, the Workers' Party newspaper, said construction was "in progress actively" and a "modern uranium enrichment plant equipped with several thousand centrifuges, to secure the supply of fuel, is operating". The article deepened suspicions with the cryptic line: "Nuclear energy development projects will become more active for peaceful purpose in the future". That indicated to observers here that the reactor as of now is developing nuclear warheads.

It's inside the same complex north of Pyongyang where analysts say an ageing "experimental" reactor has already produced enough plutonium for a dozen warheads. A South Korean foreign ministry official called North Korea's boast of its success in building the reactor "quite worrisome" and "a violation" of UN Security Council resolutions imposing sanctions after each of the North's two underground nuclear tests in October 2006 and May of last year.

If the Wikileaks cables are to be credited, China, long seen as supportive of Pyongyang because it would not wish to have a unified US-aligned Korea on its border, may not be as tolerant of such provocations as had previously been publicly understood.

Last night, the US urged China to tell North Korea that its "belligerent behaviour" must come to an end. "I think you'll see progress on multilateral discussions around this over the next few days," White House spokesman Robert Gibbs said.