At least 11 miners died in a coal mining accident in Russia’s Siberia on Thursday and an operation to rescue 35 more people stuck underground was suspended due to the risk of an explosion, the region’s governor said.

The accident happened when 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 chance of an explosion is very high. We’ve decided to suspend the search and rescue operation until the concentration of gas reduces,” Regional Governor Sergei Tsivilev said.

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

Four were in critical condition.

Before the rescue operation was halted, the governor said there was still electricity and ventilation in the mine, but that they had lost contact with some people deep underground.

At least 239 made it above ground, authorities said. They did not say what had caused the smoke. Kemerovo declared a three-day period of mourning.

In 2007, Kemerovo was the site of the worst mining accident since the 1991 collapse of the Soviet Union when an explosion at the Ulyanovskaya mine claimed the lives of more than 100 people.

In 2010, explosions at the region’s Raspadskaya mine killed more than 90 people.

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