* Gases force rescuers to retreat from West Virginia mine
* Rescue teams hopeful of returning on Thursday evening
* Worst U.S. mine disaster since 1984, at least 25 dead (Updates with new briefing; rescue attempt still on hold)
By Jon Hurdle
MONTCOAL, West Virginia, April 8 Dangerous gases forced rescue teams on Thursday to temporarily suspend a search for survivors of a massive explosion at a West Virginia coal mine that killed at least 25 workers.
Rescuers were hopeful they would be able to re-enter the Upper Big Branch mine later on Thursday evening to look for four workers missing since Monday's blast, West Virginia Governor Joe Manchin told a news briefing.
Surface crews are drilling 1,100-foot (335-metre) boreholes and using exhaust fans to further ventilate the mine owned by Massey Energy MEE.N.
"It's trending in the right direction," Manchin said of the levels of dangerous gases, including methane. "They don't have the margin of safety right now to say they can go."
Anxious families have clung to the hope that some of the miners have survived the blast and made their way to one of the mine's underground refuge chambers that are stocked with food, water and air, though officials said the odds were slim.
Those shelters offered the only escape from deadly gases in the mine, officials said. Aside from sending in rescue crews, authorities said, they were also hoping to lower a camera down a borehole to see if shelters had been used.
If the chambers had not been used, chances that the missing miners are alive "diminish tremendously," Manchin said.
The blast is the deadliest U.S. mine disaster since 1984. Eighteen bodies remained in the Montcoal, West Virginia, mine about 30 miles (48 km) south of the state capital Charleston.
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 laws governing the mining industry. Massey 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.
On Monday it was cited for two mine safety violations, which mining authorities said were unrelated to the blast.
Shares of Massey, the largest coal producer in the Central Appalachia mountain region, closed up 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.
On Thursday morning, teams of rescuers began a grueling 5-mile (8-km) underground trip into the mine, but the gases forced them to turn back, Manchin said at an earlier news conference.
"Unfortunately we have the worst scenario," he said.
The setback marked the second time rescuers were driven back by the buildup of dangerous methane and carbon monoxide. The buildup of combustible methane is often cited as a cause of mine explosions.
"You don't want to drill into that area while you have a mine rescue team underground because you get into explosive mixtures and cause more problems than what you need," said Chris Adkins, chief operating officer for Massey.
In 1984, 27 miners died in a mine fire in Utah. West Virginia was home to the worst U.S. coal mine disaster when 362 miners died at the Monongah mine in 1906. (Additional reporting by Ross Colvin; writing by Ellen Wulfhorst and Michelle Nichols; editing by Mohammad Zargham)