BEIJING (AP) — Thirteen people have been found dead after a gas explosion in a Chinese coal mine and the status is unknown of 20 others still trapped, state media said Tuesday.

Rescuers worked through the night at the privately owned Jinshangou mine in the Chongqing region where the explosion occurred before noon Monday, Xinhua News Agency reported. Two miners escaped earlier.

Xinhua previously reported 15 deaths in the explosion, but said Chongqing deputy mayor Ma Huaping lowered the death toll in a press briefing early Tuesday, saying only 13 bodies had been found so far. Local officials did not answer telephone calls from The Associated Press, and a person who answered the phone at the mine hung up when asked about the blast.

"We are still working all-out to search for the 20 missing miners, and will exert our utmost as long as there's still a ray of hope," Ma said, according to Xinhua.

Xinhua reported that the 400 workers trying to rescue more miners were being hindered by debris blocking some of the mine's passageways.

Gas explosions inside mines are often caused when a flame or electrical spark ignites gas leaking from the coal seam. Ventilation systems are supposed to prevent gas from becoming trapped.

The State Administration of Work Safety ordered an investigation into the blast, "adding that those responsible must be strictly punished." Local officials in Chongqing also ordered the temporary shutdown of coal mines producing less than 90,000 tons a year, Xinhua said.

China's mining industry has long been among the world's deadliest. The head of China's State Administration of Work Safety said earlier this year that struggling coal mines might be likely to overlook maintenance.

China is the world's largest producer and consumer of coal but has announced plans to shutter more than 1,000 outdated mines, as part of a broader plan to cut down on overproduction.