North Korea’s assertion that it has “manufactured nukes”, coupled with a declaration that it will boycott the US-led six-party nuclear negotiations for “an indefinite period”, is a coup de théâtre by an aggressively unstable regime that is internally riven and under increasing external pressure. There is no reason for this to be the coup de grâce for disarmament efforts on the Korean peninsula, provided that the demented defiance is handled sternly.

The truth about North Korea’s nuclear capability cannot be verified, but yesterday’s boast is compatible with available intelligence. Pyongyang has never before gone further than to claim that it possessed a “nuclear deterrent”, had weaponised plutonium and was planning weapons tests. Real-ity is not the strong point of a regime that is sustained by a fantastic personality cult, but the new claims highlight the unpredictability of the regime. The intriguing question, given that North Korea has conducted no nuclear weapons tests, may be whether it has made these alleged bombs, or bought them in. Last week Seoul Shinmun reported that Pyongyang had managed to purchase an off-the-shelf nuclear weapon. The report, presumably based on South Korean intelligence, may not be as bizarre as it seems, given that North Korea specialises in the bizarre.

The political, as distinct from military, significance of North Korea’s move is considerable. Since being confronted in 2002 with US evidence that it had cheated on a 1994 agreement to suspend all its nuclear weapons programmes, the regime has tried bluff, bluster and partial truths. Both forced admissions and boastful hints emanated from behind closed doors, in official negotiations or in easily retracted hints to reporters. Never before has it made a formal public statement that it possesses nuclear bombs.

The regime of Kim Jong Il may not have intended to make life more difficult for Beijing, but it has surely done so. China’s stance throughout has been that it opposes a nuclear-armed Korean peninsula, but that since it was unclear whether North Korea was even close to the point of developing actual nuclear weapons, the US and Japan should have the patience to fall in with China’s preferred strategy of gradual engagement. Only last week, US envoys were in Beijing to show the Chinese evidence of a North Korean sale of a uranium compound to Libya, and to deliver a letter, from George W. Bush to President Hu Jintao, speaking of “the greatly heightened urgency” of tackling the problem. Now that Pyongyang itself has made that case, Beijing must have been thankful yesterday that the Chinese new year holiday excused it from early comment.

Speculation on North Korea’s motives is, as Condoleezza Rice observed yesterday, a thankless occupation. On occasions North Korea has resorted to particularly belligerent rhetoric with the aim of pushing up the price of its attendance at a further round of talks; and China has played its game, bribing North Korea heavily with oil, grain and other supplies merely to turn up. But the regime’s hand is weaker now; and there is increasing, although fragmentary, evidence that it is also bitterly split. Among the six, there is closer agreement between the US and Japan on making Pyongyang sweat for its intransigence. New Japanese maritime insurance laws could all but stop trade and remittances to North Korea. The US is adamant that it will not reward bad behaviour — and this is spectacularly bad behaviour. China could bring North Korea to a standstill tomorrow if it cut off oil supplies, and knows well how close its economy is to the edge. North Korea is now extremely vulnerable to pressure. China must apply it.