China is unlikely to punish troublesome ally North Korea harshly for Tuesday's nuclear test, analysts say, even though Chinese state media had warned Pyongyang of a "heavy price" if it went ahead.

Beijing's foreign ministry expressed "firm opposition" to the blast, saying it had been carried out "despite widespread opposition from the international community" and urging the North "to honour its commitment to denuclearisation".

But the statement stopped short of condemning the test and made no mention of any possible sanctions or reprisals.

In recent weeks the state-run Global Times has issued strongly-worded editorials urging Beijing to take a tougher line on Pyongyang, saying it would have to pay dearly for another atomic explosion.

China has long supported its unpredictable neighbour for fear that instability could bring refugees flooding across the border, a US-led escalation in the region or even ultimately a unified Korea with a US military presence next door.

"I think that China is very angry about this test," said Stephanie Kleine-Ahlbrandt, Northeast Asia director of the International Crisis Group think tank.

But she added: "We won't necessarily know about any punitive measures implemented by China, and they will not necessarily deter North Korea because China is only willing to go so far. Their main concern is stability in North Korea."

Chinese trade and aid allow the impoverished nation to survive.

At once important yet uncontrollable, North Korea poses a similar quandary for China as small-nation allies did for the US during the Cold War, said Wang Dong, a Northeast Asia expert at Peking University.

"You have small allies which are behaving in a very dangerous, aggressive and provocative way, potentially with the danger of driving the US into a conflict that it did not want to get into," he said.

"China is put in a very similar dilemma."

Beijing will probably respond with limited measures coordinated with other nations, perhaps to cut off financial access, Wang said. It would keep any unilateral measures under wraps to avoid antagonising Pyongyang.

"Of course it will not acknowledge that because they do not want to publicly humiliate North Korea," said Wang. "Face is something very much taken into consideration in international relations in East Asia."

In 2006 Beijing quietly reduced the oil supply upon which Pyongyang depends, two months after the regime fired a ballistic missile and one month before it tested its first nuclear bomb.

The move only emerged later when trade data revealed it -- and Beijing subsequently stopped publicising the figures.

Ahead of the blast, US envoy Glyn Davies said the US and China had "achieved a very strong degree of consensus" on North Korea.

But China has previously worked to soften international measures. It only agreed to a UN resolution condemning December's long-range rocket launch by Pyongyang after lengthy negotiations in which it opposed stronger sanctions.

It also diluted the UN response to North Korea's second nuclear test in 2009 and was not known to have taken any unilateral measures.

Even tough punitive measures might fail to dissuade North Korea, which sees atomic arms as vital to its legitimacy and security. This view may have been reinforced after the longstanding Kadhafi regime in Libya, which surrendered its nuclear weapons, fell to Western-backed rebels in 2011.

"They've drawn the conclusion that countries that give up their nukes get slammed," said Kleine-Ahlbrandt.

Given Pyongyang's doggedness, she said, Beijing's unwavering support for the regime has generated fierce debate among Chinese policy analysts, with some arguing that the relationship brings more liabilities than benefits.

But China's overriding strategic interest in avoiding instability means its support is likely to endure, said Sarah McDowall, a senior Asia analyst with the consultancy IHS Global Insight.

"Its overarching objective really is to ensure stability in the Korean peninsula," she said.

"The relationship is going to remain strong, and China will continue to take political and economic measures aimed at propping up and supporting the North Korean regime."