At least 170 workers were trapped Sunday after an explosion tore through a central Chinese coal mine, sending smoke billowing out from air vents, the government said. Another 123 were rescued.

The blast occurred in the state-owned Chenjiashan (search) coal mine in Shaanxi province at 7:20 a.m., when 293 workers were underground, the official Xinhua News Agency said.

The blast was centered around coal pits five miles from the mine entrance, Xinhua said.

Most who managed to escape were working close to the mouth of the mine, it said, and many suffered from carbon monoxide poisoning.

Witnesses said they saw "huge amounts of thick smoke pouring from the mine's ventilation vents," hampering rescue efforts, according to the Web site of the Communist Party's People's Daily newspaper.

Staff at Chenjiashan said communications with the trapped miners had been lost, the report said.

A man who answered the telephone at the government office of Yao County, where the mine is located, confirmed that there had been an accident but refused to provide any more details.

"It's still under investigation," said the man, who refused to give his name.

A woman at the coal mine's main office who only gave her surname, Yu, said 170 workers remained trapped. She said she could not provide any details.

China's mines are the world's most dangerous, with thousands of deaths reported every year due to explosions, fires, cave-ins and flooding often blamed on lax safety rules and lack of required equipment.

The government has vowed to improve conditions and frequently orders mass shutdowns and safety checks after a fatal mine accident. But despite the crackdown, accidents still happen on a near daily basis. Chinese officials have suggested that a countrywide energy shortage may be pressuring the mining industry to raise coal production.

The worst accident in four years occurred last month, when a massive explosion in the Daping Mine (search) in central Henan province left 148 people dead.

It was sparked after mine operators failed to realize that extending the mine's shaft would greatly increase its gas level.