“We strongly urge [North Korea] to remain committed to its denuclearisation commitment, and stop taking any actions that would make the situation worse,” foreign ministry spokeswoman Hua Chunying (華春瑩) said.

READ MORE – Don’t believe the hype: Nuclear experts remain sceptical about North Korea’s hydrogen bomb claims

She said Beijing had no advance knowledge of the test, but would “make an assessment” of Pyongyang’s claim that it involved a hydrogen bomb. If confirmed, this would be the country’s fourth test of a nuclear bomb.

Its previous trials have been of atomic devices, which are less powerful.

Beijing has condemned each nuclear test and the latest development has put focus on how it will respond this time.

READ MORE: H-bomb test by North Korea sends tremors to China’s border

Many diplomats and analysts have said that China played a key role in reining in North Korea’s nuclear ambitions, but its economic support – and in particular its petroleum exports – provided an important lifeline to Pyongyang and rendered the international sanctions ineffective.

But many also believe that China’s sway on North Korea has diminished since Kim Jong-un took power four years ago. Relations between the ideological allies have cooled since then. Many analysts put this down to Kim’s authorisation of the third nuclear test, in February 2013, against Beijing’s advice.

Observers said Beijing was likely to respond to the latest test with more stringent measures, such as cooperating with the UN Security Council on another round of sanctions, but possibly not to the extent some countries would hope for due to strategic calculations.

“It’s going to be difficult for China to devise the scale of the sanctions: to impose sanctions without creating instability or even leading to the country’s collapse,” said Cai Jian, a professor of Korean studies at Fudan University in Shanghai.

READ MORE: ‘I see North Korea as one of the most evil, inhuman situations happening on earth’

Halting energy supply, for instance, would cause economic chaos and political instability in North Korea and “China would rather be cautious about it”, Cai said.

Many Chinese analysts have in recent years argued that the ­reclusive state has become more of a liability than a useful partner, but Cai said ensuring the political stability of a neighbour remained an important consideration.

“For China, North Korea’s nuclear programme itself does not pose major security threats, but rather it’s the resulting response from the US and Japan, such as enhancing their military deployment in the region, that is causing a headache for Beijing.”

There had been signs that Beijing and Pyongyang were seeking to repair ties in recent months.

In October, Beijing sent one of its most senior officials, Liu Yunshan (劉雲山), to Pyongyang to attend the 70th anniversary of the country’s ruling party. Last month, Pyongyang sent Kim’s favourite pop band, The Moranbong, on a performance trip to Beijing. But the show was cancelled at the last minute.

Yesterday’s test had dealt a blow to such efforts and would halt further high-level exchanges between the two countries, said Shi Yinhong, a professor of international relations at Renmin University in Beijing. “We can only do our best, such as to impose sanctions and pick up diplomatic ties when things are better,” Shi said.

But the cold shoulder should go only so far, Shi said. “If our relations became hostile, it would be a threat to China.”

WATCH: Chinese traffic cameras capture tremors from bomb test

North Korea announced in a statement carried by the country’s official news agency KCNA that it had detonated a miniaturised hydrogen nuclear device at 10am yesterday in a “safe and perfect manner”.

“The test means a higher stage of [North Korea’s] development of nuclear force,” it said.

Kim first claimed the country was in possession of a hydrogen bomb in December, when he visited a historical site known as the birthplace of the country’s domestic armaments industry.

At the time, Washington and some analysts had doubted whether the country had the ability to develop such an advanced weapon. Following yesterday’s test, some nuclear and military experts remained sceptical, saying the detected seismic activity was too small for a hydrogen bomb, or suggesting that the testing was ­incomplete.

South Korean President Park Geun-hye, speaking at an emergency meeting of the country’s National Security Council convened immediately after Pyongyang’s announcement, described the test as a “grave provocation”.

“The test is not only a grave provocation to our national security but also a threat to our future ... and a strong challenge to international peace and stability,” she said, calling for strong sanctions on Pyongyang.

South Korea’s defence ministry spokesman Kim Min-seok said South Korea’s military had upgraded its alert status and would “take necessary measures” in cooperation with its ally, the United States.

He said the allies had increased their joint surveillance of North Korea’s military activities.

Japanese Prime Minister Shinzo Abe said Tokyo would make a “firm response” to North Korea at the UN Security Council.

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