BEIJING — An apparent blast at a coal mine in northern China has killed 21 workers.

Four miners were rescued by emergency workers. Seven managed to escape on their own after the accident Saturday at the Wujiu coal mine outside Datong city in Shanxi province.

All of the victims died of carbon monoxide poisoning. The number of miners underground at the time of the accident was not known and rescuers are continuing to search.

News reports in China say a preliminary investigation into the accident showed it was probably caused by an explosion.

The Wujiu mine is a registered operation with an annual production capacity of 150,000 tons.

China's mines are the world's deadliest, with explosions, cave-ins and floods killing nearly 3,800 people last year.