Miners' relatives were gathering at the entrance to the Zasyadko mine desperately trying to get more information, a Reuters reporter at the scene said.
Local officials said about 70 people were working in the mine when the explosion happened. One worker, leaving the mine in dirty overalls, said he had heard that 45 people were still trapped underground, though there was no immediate confirmation of this figure, Reuters reported.
"According to preliminary information, more than 30 people were killed. Rescue workers have not yet come to the place of the explosion, they are removing the poisonous gas and then will go down," said Vladimir Tsymbalenko, the head of the local mining safety service.
The cause of the blast in the Zasyadko coal mine was not immediately clear. An explosion at Zasyadko in 2007 killed 106 people.
The sister of one miner who was in the pit at the time of the explosion, Alexei Novoselsky, stood at the entrance to the mine, in tears.
"Tell me, are there survivors? Why are you concealing the truth," she said as a local rescue services employee tried to calm her.
A welder at the mine, who gave his name as Oleg, said outside the entrance: "What I heard is that there are 45 people trapped. They got one out."
Asked about the chances of rescuing more miners, he said: "I've been down the pit for 23 years, and this is the fourth explosion that I can recall. If they didn't get them out straight away, then later they will only retrieve bodies. An explosion is a terrible thing."