The massive explosion, which Pyongyang claimed was a miniaturised hydrogen bomb, has put the region on edge and raised questions.
The massive explosion, which Pyongyang claimed was a miniaturised hydrogen bomb, has put the region on edge and raised questions about how Beijing will respond to its neighbour's latest provocation.
China "launched stern representations with the person in charge of the DPRK embassy in China", foreign ministry spokesman Geng Shuang told a regular press briefing, using an acronym for the North's official name.
"China opposes the DPRK in carrying out nuclear missile development and we are committed to denuclearisation of the peninsula. This position is well-known and the DPRK also knows this position perfectly well," he said.
"The DPRK must be very clear about that, so we hope all parties -- especially the DPRK side -- could exercise restraint and refrain from further escalating the tensions."
Geng did not say whether Beijing, which has long been hesitant to put excessive economic pressure on Pyongyang, would support further sanctions on the regime.
North Korea's announcement of Sunday's test brought strong condemnation from the international community.
The blast was five times larger than the last test a year ago, according to South Korea, and could be felt in Chinese cities hundreds of kilometres from the North's border.
Beijing is Pyongyang's only significant ally and crucial trade partner. It is considered a critical player in efforts to persuade North Korea to abandon its weapons programmes.
It has expressed strong condemnation of Sunday's test, carried out hours before Chinese president Xi Jinping was set to deliver a major speech at a gathering of BRICS nations in southern China.
The US has pushed China to take a tougher stance on North Korea.
On Sunday US President Donald Trump tweeted that he was "considering, in addition to other options, stopping all trade with any country doing business with North Korea".
Geng bristled at the suggestion.
"What is definitely unacceptable to us is that on the one hand we work so hard to peacefully resolve this issue, and on the other hand our interests are sanctioned and jeopardised," he said.
"This should not be the case and this is not fair."