Rescuers venture back into West Virginia coal mine

* Previous rescue attempt stopped by dangerous gases

* Four missing miners could be in refuge chambers

* Worst U.S. mine disaster since 1984, at least 25 dead

By Jon Hurdle

MONTCOAL, West Virginia, April 9 (Reuters) - Rescuers ventured back into a West Virginia coal mine on Friday to search for four miners missing deep underground since an explosion nearly four days ago killed 25 others.

Dangerous gases forced rescuers to retreat from the Massey Energy MEE.N mine early on Thursday until it could be further ventilated through 1,100-foot (335-metre) boreholes. A buildup of methane is often cited as causing mine explosions.

Rescue teams went back into the mine at about 12:30 a.m. EDT (0430 GMT) to head 5 miles (8 km) underground to check if the missing miners made it to refuge chambers stocked with food, water and air, though officials have warned the odds were slim.

"Now is the time to move," West Virginia Governor Joe Manchin told a news briefing, adding that the families of the miners were "relieved knowing that the process is nearing an end."

Officials said it would take two teams of eight rescuers each about 90 minutes to get within 500 feet (150 metres) of the first refuge chamber. The second refuge chamber they plan to check is a further 2,000 feet (600 metres) inside the mine.

Nitrogen is being pumped into the mine, which is about 30 miles (48 km) south of the state capital Charleston, through one of the boreholes to prevent the gases from reaching explosive levels again by neutralizing them, authorities said.

Recovery teams were also being sent into the mine to begin recovering the remaining 18 bodies. Seven of the 25 dead were recovered soon after the blast on Monday, which was the deadliest U.S. mine disaster since 1984.

"Pray for the families and the rescue workers," Massey Energy Chief Executive Don Blankenship posted on Twitter on Thursday.

U.S. President Barack Obama ordered mine safety officials to report next week on the cause of the explosion, the Upper Big Branch mine's safety record and what steps the government could take to prevent a similar disaster in the future.

Questions have risen about Massey's safety record and U.S. mining laws. Massey, the largest coal producer in the Central Appalachia mountain region, has defended its record, saying its accident rate hit an all-time low in 2009.

The Upper Big Branch mine had three fatalities since 1998 and a worse-than-average injury rate in the past 10 years, according to federal records, which also show it has been cited for more than 100 safety violations already this year.

It was cited for two mine safety violations on Monday, which mining authorities said were unrelated to the blast.

Shares of Massey closed up about 2 percent on the New York Stock Exchange on Thursday after losing more than 10 percent since the accident. Analysts predict long-term financial health for the company.

The Upper Branch mine blast is the nation's deadliest mining disaster since 27 miners died in a fire in Utah in 1984. West Virginia was home to the worst U.S. coal mine disaster, when 362 miners died at the Monongah mine in 1906. (Writing by Michelle Nichols and Ellen Wulfhorst, editing by Eric Beech)