One blast occurred late Tuesday at a private mine in Qitaihe City, Heilongjiang province, trapping 22 workers, Xinhua news agency said.

Twenty-one were confirmed dead Friday night, it said, citing provincial authorities.

Rescue efforts were hampered by debris from the blast in some of the tunnels, according to an earlier report.

In a separate incident, a mine in Inner Mongolia was struck by an explosion that left at least 17 dead, according to figures from local authorities. An unspecified number of miners were still buried under the ground and rescue efforts were underway, Xinhua said.

China is the world's largest coal producer and deadly accidents are common.

A further 33 miners were killed in a colliery explosion on October 31 in the southwestern municipality of Chongqing, and in September at least 18 were killed in a mine blast in the northwestern Ningxia region.

Officials say the number dying annually in the country's mines has fallen substantially in the past decade, to fewer than 1,000 a year.

But some rights groups argue the actual figures are significantly higher due to under-reporting in a sector with poor oversight.

Copyright AFP (Agence France-Presse), 2016