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There's only a slim chance four missing miners will be found alive in a West Virginia coal mine where 25 workers died in the worst U.S. mine disaster in more than 15 years.

State mine director Ron Wooten said the explosion occurred at about 3 p.m. ET on Monday at the Upper Big Branch Mine-South in Raleigh County, about 50 kilometres south of Charleston.

Emergency crews were sent to the mine, near the town of Montcoal, but toxic gases forced rescue teams from the mine early Tuesday.

The mine, which has a long history of safety violations, is operated by Performance Coal Co., a subsidiary of Massey Energy Co. 

Safety officials said some miners were killed by the blast while others died because they breathing in toxic air. Eleven bodies have been recovered and identified, and at least 14 bodies are still in the mine, officials said.

The death toll is the highest in a U.S. mine disaster since 1984, when 27 died in a fire at Emery Mining Corp.'s mine in Orangeville, Utah. If the four missing men are found dead, bringing the toll to 29, it will be the most mining deaths since a 1970 explosion killed 38 at Finley Coal Co. in Hyden, Kentucky.

The cause of the blast is still unknown.

Officials said the situation for those missing is dire but that rescuers would try to go back after the mine is ventilated.

Search delayed

Gov. Joe Manchin said at a Tuesday news conference that drilling on at least one of four ventilation holes would begin before Tuesday night and not finish until Wednesday night. 

"It's going to be a long day and we're not going to have a lot of information until we can get the first hole through," Manchin said.

The drills need to bore through about 335 metres of earth and rock, he said.

"All we have left is hope, and we're going to continue to do what we can," said Kevin Stricklin, an administrator for the federal Mine Safety and Health Administration. "But I'm just trying to be honest with everybody and say that the situation does look dire."

President Barack Obama offered his support to the miners' families at an Easter prayer service on Tuesday.

"I want to send my deepest condolences, our thoughts and prayers to the families and friends of the workers who lost their lives after an explosion took place at a West Virginia mine yesterday," Obama said.

Obama said he spoke with Manchin, saying he would offer federal help to the state if the governor needed any.

Local resident Grace Lafferty spent Monday night at the mine's site, offering support to the victims' families.

"It was just devastating to watch it unfold not knowing who got out and who were still in the mine," Lafferty told CBC News. "There are really no words to describe what it was like."

Massey Energy's sprawling Upper Big Branch mine has a history of violations involving the ventilation of combustible methane gas.

Last year, the mine faced 458 safety violation citations, and 50 of those were classified as "unwarrantable failures to comply," according to the Mine Safety and Health Administration.

At the time of Monday's explosion, the mine was facing more than $150,000 US in fines for pending safety violation charges.

In 2006, Massey Energy was fined $1.5 million for 25 violations that inspectors concluded contributed to the deaths of two miners who were trapped in a fire in another West Virginia mine owned by the company.

With files from CBC News