One American soldier and three Afghan fighters were killed Saturday during an attack on regrouping Taliban and Al Qaeda forces in the eastern mountains of Afghanistan, according to reports from the Pentagon in Washington and U.S. Central Command in Flordia.

Several others were wounded in the battle, which was temporarily suspended Saturday afternoon to allow U.S. bombers to soften enemy positions. More than 80 "thermobaric" bombs were dropped on the area before fighting resumed.

It was unclear how many Al Qaeda and Taliban troops died. The American and Afghans were killed by enemy fire.

The casualties came during the largest U.S.-led ground operation in the anti-terror campaign since January, military officials said. A U.S. Defense official said 1,500 Special Forces troops, Afghan fighters, and members of the Army's 101st Airborne were assembled for the battle.

Intelligence officials said between 600 and 1,500 Al Qaeda and Taliban were regrouping there. Afghan fighters interviewed in Gardez said the Americans told them there were about 4,000 Al Qaeda and Taliban warriors holed up in the eastern mountains.

The death of the serviceman Saturday was the first U.S. casualty from hostile fire in a ground engagement since Operation Enduring Freedom began. The previous deaths from hostile fire came during a prison uprising and in a sniper incident near Khost.

Other previous U.S. casualties have resulted from friendly fire or accidents.

Saturday's battle plan called for a combination of American Special Forces and Army troops fighting alongside Afghan allies on the ground with U.S. bombing support from the air, said Pentagon officials, speaking on condition of anonymity.

By comparison, big gains in the war from October through December came mostly in operations in which Afghan allies fought on the ground with small teams of U.S. special forces calling in bombing targets for warplanes.

Even before word of the casualties, the new operation reinforced what the Pentagon long has maintained – that the war against terrorism in Afghanistan was far from over.

The most recent and largest sustained bombing was in January, when warplanes bombed caves and exploded enemy weapons and ammunition for more than a week at Zawar Kili.

The last known ground operation was on Jan. 23 when U.S. special forces raided on a compound where the United States mistakenly believed enemy figures were holed up. The Pentagon has said 16 people who turned out not to be to Al Qaeda or Taliban were killed when they resisted.

Since then, there have been small unpublicized raids, said a U.S. defense official, speaking on condition of anonymity.

Saturday's attack also marked the first use of newly-developed "thermobaric" bombs. The 2,000-pound bombs use two explosions to achieve their desired effect. First, the bomb penetrates a cave or tunnel and scatters explosive dust. A fraction of a second later, a second, larger explosion sucks the oxygen from the area.

The name is descriptive; "thermo" refers to the heat of the blasts caused by the warheads, and "baric" to the changes in barometric pressure. The powder-based explosive makes the bombs more effective in closed spaces than liquid bombs.

The benefit of the thermobaric bomb, however, comes at a price. While it collapses caves and tunnels, eliminating the need for troops to enter the often booby-trapped areas, it also buries any evidence inside, making it more difficult to tell if a mission has succeeded.

Coalition troops continue to gather intelligence on pockets of resistance, seizing material and interrogating people during these raids, the official said.

U.S. officials and Afghan sources estimate 4,000 to 5,000 foreigners who fought for the Taliban and Al Qaeda remain inside Afghanistan.

"We've said all along that it is not over ... in Afghanistan and that for some time there would be pockets of resistance," Pentagon spokeswoman Victoria Clarke said Friday.

The Associated Press contributed to this report.