Rescuers searched for 20 missing coal mine workers on Tuesday, a day after an explosion killed at least 13 workers and spread poisonous gas into parts of the mine in southwestern China.

“Two miners managed to escape from the Jinshangou coal mine, where firefighters and militia members were digging through tunnels blocked by debris after Monday’s blast,” reported China’s state broadcaster CCTV .

“The rescue team has carried out three underground operations so far,” said Chongqing Deputy Mayor Mu Huaping on Tuesday.

“We have found that some tunnels have collapsed and the amount of poisonous gas has exceeded the alarm level at some parts,” he added.

“Chongqing officials ordered the temporary closure of some coal mines in the area while investigators look into the accident at the Jinshangou mine,” reported the official Xinhua news agency.

Coal accounts for almost two-thirds of China’s energy consumption, but its mines are among the world’s deadliest, due to lax enforcement of safety standards.

Last month, an explosion in a coal mine killed 20 and trapped more than 30 workers underground in the country’s central region, the Xinhua News Agency reported.