Kunming - At least 20 miners were killed on Thursday in an accident in Southwest China's Yunnan province, local authorities said, and another 23 were still missing late in the day.

Rescuers on Thursday enter a shaft at the Sizhuang Coal Mine in Shizong county, Yunnan province, where a coal and gas outburst killed 20 people and trapped 23. [Photo / China Daily]

The deadly accident occurred at 7:20 am when an outburst of coal and methane gas hit the Sizhuang Coal Mine in Shizong county, the State Administration of Work Safety said in a brief statement.

Forty-three miners who were working in the shaft were trapped immediately after the outburst.

Such an outburst usually occurs when high pressures underground make coal and methane gas billow up and fill mine tunnels.

By 3 pm on Thursday, at least 20 miners were confirmed dead. Rescuers were struggling to reach another 23 and were still nearly 100 meters from the position of the trapped miners, Tan Xiaopeng, head of the local fire brigade, said on Thursday afternoon.

The search and rescue operation faces great difficulties, said Tan, because gas intensity in the shaft remains so high that it could easily ignite.

The local government sent more than 30 ambulances and 100 medical personnel. The number of doctors and nurses will soon rise to more than 300.

Local authorities said that the mine had been run illegally, as its safe work license was revoked on Nov 28, 2010, but it later resumed production.

Luo Lin, head of the State Administration of Work Safety, ordered local authorities to try every means to save the trapped miners.

A rescuer enters a shaft at the Sizhuang Coal Mine in Shizong county, Yunnan province. [Photo /Xinhua]

Zhao Tiechui, deputy head of the State Administration of Work Safety, and Peng Jianxun, deputy director of the State Administration of Coal Mine Safety, have been dispatched to Qujing to oversee the rescue efforts.

The private mine, with an annual output capacity of 90,000 tons of coal, has a relatively poor safety record, according to the Yunnan Administration of Coal Mine Safety.

In April 2005, a fire caused by gas rocked the mine when it was under construction to expand its capacity, killing four miners in the shaft.

The accident resulted from lack of safety measures to prevent underground gas buildup, the coal mine safety administration said later in an investigation report.

Xinhua contributed to this story.