Rescuers worked to free 21 trapped workers from an illegal coal mine in China on Friday.

The Xiaoyi city mine, in China's northern coal-producing Shanxi province, flooded late Wednesday, trapping the workers underground.

Rescue efforts were hampered by the small and concealed entrance to the illicit mine, as well as a lack of maps, authorities said. Low temperatures and strong winds have also hampered efforts, state media reported. 

Chinese authorities have cracked down on illegal mining operations in recent years, which have surged amid increasing coal prices. China relies on coal for at least 60% of its energy generation, and has been hit by recent shortages.

At least 20 miners were rescued by late Friday, according to state media.

About 400 rescue workers were involved in the efforts, and pumps were dispatched to help drain the mine.

State broadcasters showed the orange-clad workers carrying people out on stretchers.

Authorities have detained at least seven people, and were searching for others, including the mine's owner, according to local media. Meanwhile, three top city officials were suspended, the local government announced.

Mining accidents are a frequent occurrence in China, with poor safety enforcement. In September, 19 miners died during in one collapse, while in January, a group of miners was trapped hundreds of meters underground in eastern China.

Last week, China's mine safety body said there had been 59 fewer mine accidents so far this year, with 336 incidents, thanks to improved workplace safety. However, it said there could be a spike in December as mines ramped up their end-of-year production.

aw/msh (AFP, AP)