China, North Korea's long-time ally, has said it "firmly opposed" Pyongyang's nuclear test and hinted at sanctions, with analysts saying their already strained relations could worsen.
Chinese Foreign Ministry spokesman Hua Chunying said at a media briefing yesterday that the North had conducted the test despite international opposition.
"We strongly urge North Korea to remain committed to its denuclearisation commitment and stop taking any actions that would make the situation worse," she said.
A commentary by the official Xinhua news agency condemned the "highly regrettable" move.
Ms Hua later said Beijing had no prior warning of the test and would summon the North Korean ambassador and senior officials "to lodge our solemn representations".
Asked about possible sanctions, Ms Hua said Beijing would fulfil its obligations, pending any decision by the United Nations Security Council, which held an emergency session last night.
But she refuted suggestions that the test reflected a failure in China's efforts to get North Korea to give up its nuclear weapons, saying instead that it showed that the "only practical way" to resolve the issue was through the six-party talks.
The talks, stalled since April 2009, bring together the two Koreas, the United States, Russia and Japan to achieve denuclearisation on the Korean peninsula, and was China's brainchild.
But Chinese analysts said that the nuclear test made it trickier for China to restart the talks, given that North Korea would want to be recognised as a nuclear power and that Western countries were unlikely to do so.
Sino-Korean relations expert Yu Yingli told The Straits Times that the test was another blow to the bilateral relationship, coming shortly after a North Korean girl band abruptly cancelled its performances in Beijing last month.
Under President Xi Jinping's leadership since late 2012, China has reacted more critically towards North Korea over its provocative actions.
The North had conducted three nuclear tests previously, in 2006, 2009 and 2013. The one in 2013 prompted China to summon the North Korean ambassador, the first time it had done so.
Mr Xi and North Korean leader Kim Jong Un, who took power in late 2011, have yet to meet.
"We are unlikely to see the leaders' meeting take place soon. China could also suspend economic and trade cooperation with the North," said Dr Yu of the Shanghai Institute of International Affairs.