China, North Korea's key diplomatic protector, said Wednesday it "firmly opposes" Pyongyang's nuclear test, adding it was carried out "irrespective of the international community's opposition".
"We strongly urge the DPRK side to remain committed to its denuclearisation commitment, and stop taking any actions that would make the situation worse," foreign ministry spokeswoman Hua Chunying told a regular briefing, using the North's official name.
She added that China would summon the North's ambassador and launch "solemn representations" over the nuclear test, Pyongyang's fourth in total and its first since a 2013 experiment that was condemned by Beijing.
China is a close ally of the isolated state, but relations have become more strained in recent years, in part because of North Korea's persistence with its nuclear programme in the face of international condemnation.
North Korean leader Kim Jong-Un has yet to visit Beijing, after coming to power following the death of his father four years ago.
Beijing was not aware of Wednesday's test in advance, Hua said, adding that China would "make our assessment" of Pyongyang's claim it was a hydrogen bomb.
China is the sponsor of the long-stalled six-party talks, which also bring together the two Koreas, the US, Russia and Japan, and Hua called them the "only practical way to resolve the relevant issue".