China strongly condemned a new nuclear test by the Democratic People's Republic of Korea on Sunday, vowing to "unswervingly push forward the denuclearization of the peninsula".

Pyongyang said earlier in the day that it had successfully detonated a hydrogen bomb. It was the country's sixth nuclear test.

"Today, the Democratic People's Republic of Korea once again conducted a nuclear test, despite widespread opposition from the international community. The Chinese government expresses firm opposition and strongly condemns this," the Foreign Ministry said in a statement on its website on Sunday afternoon.

It is China's firm position and a common aspiration of the international community that denuclearization of the Korean Peninsula should be achieved to maintain the nuclear nonproliferation regime and maintain peace and stability in Northeast Asia, the statement said.

"We strongly urge the DPRK to face the international community's resolve on denuclearization of the peninsula, and to concretely comply with resolutions of the UN Security Council, stop taking wrong actions that will cause the situation to deteriorate and undermine its own interests, and come back to the track of resolving the problem through dialogue," the ministry said.

"China, together with the international community, will comprehensively and completely implement the resolutions of the Security Council concerning the DPRK, unswervingly push forward the denuclearization of the peninsula and unswervingly safeguard peace and stability," it said.

The China Earthquake Administration said in a statement issued on its website that a magnitude-6.3 tremor occurred in the DPRK at 11:30 am. It could have been an explosion, the statement said.

China's National Nuclear Safety Administration said it initiated its emergency response plan for nuclear radiation testing at 11:46 am.

It said radiation monitoring sites in Heilongjiang, Jilin and Liaoning provinces in Northeast China, as well as in Shandong province not far from the DPRK, are all in normal operation.

The administration has been carrying out regular radiation surveillance at the border.