China should exact a heavy price from North Korea if it carries out a planned nuclear test, state-run media said yesterday in an unusually strongly worded call for action by Pyongyang's closest ally.

Beijing, which has stressed calm and restraint as its unpredictable neighbour prepares to conduct its third nuclear test, has tended to avoid punitive measures against it, partly for fear of provoking regional instability.

"If North Korea insists on a third nuclear test despite attempts to dissuade it, it must pay a heavy price. The assistance it will be able to receive from China should be reduced," the Global Times said in an editorial.

"China is never afraid of Pyongyang," it said. "If Pyongyang gets tough with China, China should strike back hard, even at the cost of deteriorating bilateral relations.

Beijing was "willing to maintain the Sino-North Korean friendship" it said, "but Pyongyang should do the same".

"China shouldn't be taken hostage by North Korea's extreme actions," added the article, which appeared in both the English- and Chinese-language editions of the paper.

Last month, Beijing's foreign ministry distanced itself from a similar Global Times editorial that threatened a cut-off of assistance if the test went ahead, calling the claim "only the opinion of the media".

North Korea vowed to conduct the test after the UN Security Council condemned its December 12 rocket launch in a resolution that was the product of extensive negotiations with China.

Beijing is seen as fearing the consequences of a North Korean collapse, which could send an exodus of refugees across the border and potentially lead to a reunified, US-allied Korea on its border. China has acted as Pyongyang's main benefactor since the 1950-53 Korean war, providing vital diplomatic support and economic ties.