BEIJING (AP) — Hundreds of rescuers were working Thursday to save 23 Chinese coal miners trapped underground by a gas leak that killed 20 others.

It comes less than a week after dozens of miners were pulled safely from a cave-in at another Chinese coal mine. Eight died in that accident.

Thirty firemen, 100 rescuers and 300 medical workers were on site at the Sizhuang Coal Mine, the official Xinhua News Agency said, citing Yunnan province's emergency response office.

The accident happened about 6:30 a.m. at the mine in the city of Qujing in Yunnan, southwest China. Conditions inside the mine were unknown.

An initial investigation found that the gas leak occurred at one platform inside the shaft, and the gas later spread to another platform, trapping 43 miners working on both platforms, Xinhua reported.

More information on the accident and rescue attempt was not available as local authorities could not be reached by phone Thursday afternoon.

China's coal mines are the deadliest in the world, although the industry's safety record has improved in recent years as smaller, illegal mines have been closed. Annual fatalities are now about one-third of the high of nearly 7,000 in 2002.

Last week, 52 miners were rescued from a mine in Henan province after a cave-in.

The rescue was the biggest in the country since April 2010, when 115 miners were pulled out alive after being trapped for eight days in a mine in northern China.