At least 21 miners died when a roof collapsed in a coal mine in northern China, officials say.

Sixty-six miners were rescued after the accident on Saturday at the Lijiagou mine near the city of Shenmu in Shaanxi province.

The cause of the collapse is under investigation, the official Xinhua news agency reported.

Mining accidents in China are quite common despite efforts to improve safety.

The Lijiagou mine is operated by the Baiji Mining Company, Xinhua said. No further details of the incident were available.

Last October, 21 coal miners were killed when an underground rock fall blocked a shaft in eastern Shandong province.

According to the latest figures from China's National Coal Mine Safety Administration, there were 375 deaths in coal mines in 2017, a fall of about 28% on the previous year.

In a statement last January, the bureau said the "situation of coal mine safety production is still grim" despite improvements.