An explosion at a coal mine in southern West Virginia killed 25 miners and left four others missing on Monday, officials said.

The explosion occurred around 3 p.m. (1900 GMT) at the Upper Big Branch Mine of the Massey Energy Company when a total of 29 miners were underground, Ron Wooten, director of West Virginia Office of Miners' Health Safety and Training, said.

Over a phone talk with West Virginia Governor Joe Manchin, U.S. President Barack Obama extended his condolences, vowing that the government is ready to offer any aid to the rescue effort.

However, rescue efforts had been halted due to a rising methane gas level in the mine, reports said.

Fortunately, the mine has caches of extra oxygen along emergency escape routes. It also has airtight refuge chambers to provide enough air to keep miners alive for four days.

But an official of the mine safety administration said the miners probably had not successfully reached the refuge chambers because the network near the victims and the missing workers was likely destroyed in the explosion.

Massey Energy, the fourth largest coal producer in the United States according to revenues, operates 35 underground and 12 surface mines in West Virginia, Kentucky and Virginia, according to the company website.

The accident is the worst mine disaster in the country since a mine fire in Utah killed 27 people in 1984.

Mine deaths are relatively not common in the United States. Last year, the number of U.S. miners killed on the job was 34, 18 of them died in coal mines. Notable accidents in recent years include West Virginia's Sago Mine methane explosion, which killed 12 miners in 2006. Six also died in Utah's Crandall Canyon mine cave-in in 2007.