The death toll from a Pakistan mine explosion rose to 21 on Monday and there was little chance of survival for the 31 men still trapped, a senior mining official said.

Poisonous methane gas was hampering the work of rescue workers, who have been working to save the men since an explosion early Sunday caused the mine to collapse.

"We are trying our best to rescue as many workers as we can, but the chance of finding survivors is bleak now," said Iftikhar Ahmed, a top mine inspector. Many of the miners they have found dead so far suffocated from the gas, he said.

The mine is in Baluchistan province, some 25 miles (40 kilometers) east of the provincial capital, Quetta.

The mine was declared dangerous two weeks ago, but the warning was ignored, said Ahmed. The mine is owned by the state-run Pakistan Mineral Development Corporation but leased to a contractor, he said.

Rescue workers found six bodies early Sunday but had to suspend their efforts because of the methane gas, said Ahmed. They found the 15 more bodies overnight when they resumed their work, he said.