Kabul, Afghanistan ? At least one American and three Afghan government fighters were killed in a major new offensive launched Saturday against an al-Qaida stronghold in the remote mountains of southeastern Afghanistan, according to U.S. and Afghan officials.

Heavy U.S. bombing began just before daybreak in Paktia province and continued Saturday night as U.S. jets attempted to flush al-Qaida soldiers from their mountain redoubt.

An Afghan commander in the region, Wazir Khan, said he believed that two or more Americans and three Afghans had been killed in the previous 24 hours. One of the Americans was killed Friday night and the other during Saturday’s operation, he said. One was in a Datsun pickup when he was fired upon by an al-Qaida sniper, he said.

In addition, he said, at least three Americans and 27 Afghans had been injured by early Saturday evening.

“The area is very well fortified and is high in the mountains,” said Wazir Khan, who spoke from the front lines. “It’s very difficult, and I can’t say how long this operation will take.”

Citing the ongoing combat, officials at U.S. Central Command in Tampa, Fla., said they were not at liberty to discuss details of the fighting or circumstances surrounding the casualties. The U.S. officials said in a prepared statement that “initial reports are that one U.S. service member and two Afghan Forces have been killed. An unspecified number of U.S. and Afghan Forces have been injured.”

Thermobaric bombs used

The events marked the first major U.S. bombing in Afghanistan since a January attack on the al-Qaida cave complex at Zhawar Kili, which is also in Paktia but far closer to the Pakistan border. Pakistan was reported to have sealed its common border with Paktia province Saturday to prevent escape by any al-Qaida members fleeing the fight.

For the first time Saturday, U.S. forces used a recently developed weapon, the 2,000-pound BLU-118B, or “thermobaric” bomb, said Navy Cmdr. Dave Culler, a spokesman for the U.S. Central Command.

A Navy fighter plane dropped the bomb, which can penetrate caves without causing much external damage, on a cave complex in the region. Because the bomb creates a blast of high pressure inside a cave, it is believed to be a particularly effective weapon in places where chemical or biological agents might be stored. A thermobaric bomb is designed to incinerate such agents rather than disperse them into the atmosphere, Culler said.

Culler declined to comment on whether U.S. military forces believed that chemical or biological weapons were stored in the cave complex.

Al-Qaida stronghold

The area under siege is a rugged, mountainous and desolate region about a six-hour drive south of Kabul, the Afghan capital, in Paktia province, which borders Pakistan. The besieged territory lies between the city of Gardez in northern Paktia and the city of Khost in the southern part of the province.

The area was known to be heavily populated by Arabs, who make up most of al-Qaida, before the fall of Kabul to Northern Alliance troops in mid-November. After that, many more Arabs came through the region as they fled south.

It has long been suspected that a number of these al-Qaida members, as well as some Taliban, stayed in the southeastern border provinces, where they have been creating pockets of resistance to both the American military campaign and the new Afghan interim government. The environment has been relatively sympathetic, since the Arabs and Taliban had ties to the local population and were familiar with the difficult mountain terrain.

In their prepared statement Saturday, U.S. military officials said that about 6:30 a.m. in Afghanistan, “Afghan and coalition forces began fighting al-Qaida terrorists and non-Afghan Taliban south of Gardez.”

Speaking from Tampa, Navy Cmdr. Culler said officials would not discuss details of the fighting because of concern that such disclosures could further jeopardize American and Afghan forces.

U.S. Air Force Brig. Gen. John W. Rosa Jr. told reporters Friday that “hundreds” of al-Qaida and Taliban had been located in or around Paktia province. Rosa also said that eliminating the pockets of al-Qaida and Taliban trying to regroup remained a primary mission of the United States.