Beijing: An explosion at a coal mine in northeast China has killed 21 people, with rescuers attempting to reach the sole survivor after three days trapped underground, state media reported on early Saturday.

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

Twenty-one of them had been confirmed dead Friday night, it said, citing provincial authorities.

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

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

In November 33 miners were killed in a colliery explosion 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.