Coal dust caught fire in a ventilation shaft in the Listvyazhnaya mine in the snowbound Kemerovo region early on Thursday, filling the mine with smoke, the TASS news agency cited local emergency services as saying.

“The search and rescue work has been temporarily suspended due to the threat of an explosion,” an unnamed emergency services spokesperson was quoted by the RIA news agency as saying.

Eleven people died and 35 others were still underground, Regional Governor Sergei Tsivilev said, according to RIA.

Dozens were being treated in hospital, at least some of them with smoke poisoning. Four were in critical condition.

Rescue workers and ambulances could be seen arriving at the mine’s compound in video footage, with police huddled outside as it snowed in the region roughly 3,500 km (2,175 miles) east of Moscow.

Some 285 people were inside the mine when smoke spread through the ventilation shaft, the emergency ministry said. At least 239 made it above ground, authorities said. They did not say what had caused the smoke.

The Kremlin said it hoped the miners who were still underground would manage to get out and that President Vladimir Putin had ordered the emergencies minister to fly out to the region to help with the operation.

Tsivilev said there was still electricity and ventilation in the mine, but that contact had been lost with some people deep underground.

“For now there is no heavy smoke, so we hope that there is no fire,” Tsivilev said in video comments shared on his Telegram channel. “We have no communication lines with these people, the underground communications system is not working.”

The Investigative Committee law enforcement agency’s regional branch said it had opened a criminal case into negligence that had caused loss of life.

“According to preliminary data, a number of workers suffered from smoke poisoning. The number of victims is being specified,” it said in an earlier statement.

The mine is part of SDS-Holding, owned by the privately held Siberian Business Union. The union had no immediate comment.

By Alexander Marrow and Tom Balmforth.

(Reporting by Anton Kolodyazhnyy, Alexander Marrow, Anastasia Lyrchikova and Gleb Stolyarov; Writing by Alexander Marrow/Tom Balmforth; Editing by Stephen Coates and Bernadette Baum)