Rescuers are racing to dig through a sludge-flooded tunnel to reach 19 miners who are trapped in a collapsed coal mine in south China for the fourth day, sources with the rescue command center said Tuesday.

By 9:00 a.m., rescuers reached half way of the 25-meter tunnel that is studded with sludge of mud and coal, said Su Fuchao, general manager of the coal mine company.

The tunnel is thought to be closest to six miners who have the highest chance of survival, Su said.

The six miners were initially estimated as being trapped 320 meters deep.

Rescuers cleared 12.5 meters of the tunnel after one day's effort.

Su said the rescue work faces with great difficulties as the density of toxic gas inside the shaft is not stable.

Rescue efforts were suspended due to high levels of toxic gas Monday morning, but were resumed later that day.

Also, the tunnel's sludge slowed the rescue operation.

"We are trying to clear off a giant rock now, but we can only progress a few centimeters per hour," Su said.

The collapse occurred midday Saturday in the suburbs of Heshan City in Guangxi Zhuang Autonomous Region.

Forty-nine of 71 miners managed to escape when the coal mine collapsed, and rescuers have retrieved three bodies of the dead miners.

Initial investigations indicate that the collapse was caused by continuous heavy rains.