China summoned the North Korean ambassador on Tuesday to express "firm opposition" to Pyongyang's nuclear test and said the North had gone ahead with the blast "despite widespread opposition from the international community".
"We strongly urge the DPRK (North Korea) to honour its commitment to denuclearisation, and not to take any actions which might worsen the situation," the foreign ministry said in a statement on its website, using the North's official name.
China is North Korea's most important backer, providing it with trade and aid that have enabled the state to survive for decades since the end of the Korean War.
But even though Chinese state-run media urged a tougher line in the run-up to the test, the foreign ministry statement contained no threat of any sanctions or reprisals.
"The Chinese government calls on all parties to respond calmly, solve the problem of denuclearisation of the peninsula through dialogue and consultation within the framework of the six-party talks," it said, in a reiteration of Beijing's longstanding position.
North Korea's ambassador to China was summoned to the ministry, it said later. Foreign Minister Yang Jiechi made "solemn representations", telling him of Beijing's "strong dissatisfaction and firm opposition" to the test.
In a commentary China's official Xinhua news agency said the explosion was an attempt by a "desperate DPRK" to keep a perceived external threat at bay.
"At a superficial level, it was Pyongyang that has repeatedly breached UN resolutions and used its nuclear programme as a weapon to challenge the world community, which was considered to be unwise and regrettable," it said.
But it added: "In reality, the DPRK's defiance was deeply rooted in its strong sense of insecurity after years of confrontation with South Korea, Japan and a militarily more superior United States."
It also urged calm and restraint, calling on "all relevant parties to... accommodate each other's concerns so as to properly manage the current crisis".