China should exact a "heavy price" from North Korea if an imminent nuclear test goes ahead, state-run media said Wednesday in their strongest call yet, but analysts say Beijing appears unable to restrain its wayward ally.

The state-run Global Times raised the prospect that the relationship founded on the battlefields of the 1950-53 Korean War, and which Pyongyang has relied on ever since, "might break down" over the issue.

That would "be of no benefit to Pyongyang", it said. "North Korea would face an even worse situation, but China could find some ways to compensate for geopolitical losses."

The article appeared in both the English- and Chinese-language editions of the paper.

But analysts and diplomats said that despite its discontent Beijing was unwilling to carry out meaningful action, leaving the media as its only theatre to display its frustrations.

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

The Global Times -- which is owned by the People's Daily, the official mouthpiece of the ruling Communist Party -- has previously urged Beijing to cut off aid if it goes ahead with the blast.

But the threats contrast with Beijing's official position of repeatedly urging calm and restraint, and avoiding punitive measures to prevent regional instability.

China is widely 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.

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

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

Stephanie Kleine-Ahlbrandt, the Beijing-based Northeast Asia director for the International Crisis Group, said that "editorials are a great way to let off steam".

Although Beijing has long maintained its stance of supporting its unpredictable neighbour, she said, the media provides a way to vent the views of those arguing for a tougher tack.

"They are really upset," she said. "But there's a higher-order priority."

Avoiding instability was China's main concern in the Korean peninsula, she said.

"Even if they diminish the assistance, they are not going to do it enough to cripple the regime or make a difference," she said. "Beijing is simply afraid of pushing the regime too far."

Beijing provides substantial economic support to Pyongyang through cross-border trade and investment, while aid constitutes "just one part of the relationship", she added.

The foreign ministry did not immediately respond to requests for comment on reports in South Korea that Pyongyang's ambassador had been summoned for talks "several times".

A car bearing diplomatic plates and carrying a North Korean flag was seen entering the foreign ministry Tuesday.

A Western diplomat in Beijing said: "I don't think the Chinese are going to change their policy, even if there is a lot of frustration here about the fact that they can't persuade Pyongyang."

The foreign ministry distanced itself from the editorials, with spokeswoman Hua Chunying saying that Global Times pieces "are perhaps not that in line" with its positions, and repeating China's call for calm and restraint.

China has acted as Pyongyang's main benefactor since the Korean War, providing vital diplomatic support and economic ties to one of the world's most isolated regimes.