There were over 400 miners underground when the explosion shook the Lenin mine, owned by Russian mining giant Mechel, in Mezhdurechensk, in the south of the Kemerovo Region at 11:10 local time (03:30 GMT) on Tuesday.
Seventeen miners were hospitalized with burns and other injuries after the explosion. Four out of the 17 miners are still in intensive care. A fire caused by the blast has yet to be extinguished.
"Poor industrial safety was a major cause of the accident," Rostekhnadzor said in a statement.
However, the official results of the probe into the accident have yet to be announced, said Roman Ivanyutenko, a local Rostekhnadzor official.
"A commission looking into the causes of the accident will announce its official results no earlier than in the next 15 days," he said.
The watchdog said five coalmine accidents had taken place in the Kemerovo Region since the start of 2008, including a cave-in at the Lenin mine on May 30, which killed five people.
Tuesday's blast occurred as Mechel's share price plummeted dramatically in New York and Moscow after Prime Minister Vladimir Putin claimed on Thursday that the company was selling raw materials at twice the price domestically as it was abroad. This was coupled with fresh accusations of tax evasion against the company by Putin on Monday.
Mechel CEO Igor Zyuzin, who was admitted to a Moscow Medical Academy clinic a day before Putin's initial criticism of the company, has already visited the mine to inspect the clean-up operation and the provision of medical assistance to those miners injured in the blast.