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SURMAD, Afghanistan (AP) _ U.S. bombers blasted the cavernous mountains of eastern Afghanistan for a third day Sunday, pressing a new offensive against al-Qaida and Taliban fighters believed to be regrouping there.

One American and three U.S.-allied Afghans were killed Saturday in the opening day of a ground offensive that accompanied the air campaign, the Pentagon said. An Afghan doctor at the Gardez hospital said at least six Americans were injured.

Sunday's assault involved mostly airstrikes, not ground attacks, with frequent bombing runs over the Shah-e-Kot mountain range 20 miles east of Surmad and the Kharwar range to the west in Logar province.

The bombardments sent thick black plumes of smoke above the snowcapped peaks and shook the ground in Surmad, where a constant stream of B-52 bombers streaked overhead.

``They have been dropping very heavy bombs all night, this morning, yesterday,'' said schoolteacher Rehmatullah as he sat on the cement steps of his newly built school in Surmad.

The bombardment was part of the largest known joint ground offensive of the war. However, Saturday's ground attack appeared to have made little headway in dislodging Taliban and al-Qaida fighters who fought back with artillery, mortars and heavy machine guns, Afghan officials said.

``Firefights have been intense at times in heavy combat action,'' Maj. A.C. Roper, spokesman of the 101st Army division in southern Kandahar, told reporters Sunday.

Roper said the coalition forces were operating in cold, snow-covered mountainous terrain ranging from 8,300 to 11,600 feet above sea level.

A U.S. defense official said a U.S.-led force of 1,500 Afghan allies, U.S. Special Forces and troops from the Army's 101st Airborne assault troops had assembled for Saturday's battle. Australian and Canadian troops also took part.

The Afghan allies approached the front from three different directions, some of them using pickup trucks rented for $200 from the bazaar in the Paktia provincial capital of Gardez, Afghans said.

About 600 fighters accompanied by at least 40 U.S. soldiers approached from Gardez, north of Surmad, said Safi Ullah, a member of the Gardez town council, or shura. Another 400 Afghans came in from Khost to the west, and an undisclosed number came from Paktika province to the south.

At least 70 vehicles carrying American and Afghan forces snaked around the back road behind Surmad to reach the front, said Rehmatullah, the schoolteacher.

A Surmad village elder, who identified himself only as Mohammed, stroked his unkempt red henna-streaked beard and pointed toward a stand of trees near the Shah-e-Kot mountain range, where the offensive was launched.

After the ground attack stalled, U.S. planes late Saturday dropped newly developed bombs designed to send suffocating blasts through cave complexes, military officials said. The ``thermobaric'' bombs were tested in December and officials said in January that they would be rushed to the region for the war.

More than 80 bombs of different types were dropped Friday night and Saturday in the snowy terrain, the U.S. military's Central Command in Florida said.

During Sunday's assault, three Chinook helicopters flanked by two jets zoomed toward the Shah-e-Kot range, ferrying ammunition and supplies to American forces still in the hills, said local commander Abdul Matin Hassan Kheil.

``You can see it is a big operation,'' he said, noting that among the al-Qaida fighters entrenched near the front were Arabs, Chechens and Pakistanis. ``I think one month we will need, inshallah,'' or God willing.

U.S. aircraft also dropped pamphlets written in Afghanistan's Pashtun and Dari languages on the arid plains near the site of the assault, warning that the days of al-Qaida and the Taliban were numbered and urging residents not to support them.

``Stop resisting. End your fight. Otherwise, you are finished,'' statements in the pamphlets declared. They urged residents to inform coalition forces against regrouping al-Qaida and Taliban troops and featured a picture of a Taliban fighter armed with a Kalashnikov sitting on a truck.

Afghan forces broke off Saturday's attack in early afternoon and withdrew back to Gardez.

Fighters recovering Sunday at Gardez hospital described the operation as two-pronged, with one group of Afghan troops and U.S. Special Forces launching a frontal attack and a second attempting to ambush from the rear.

Fighter Raza Khan said the American was killed when a pickup truck he was riding in was hit by a mortar shell.

Six injured Americans were airlifted out of the area by helicopter, said a doctor at Gardez hospital, Najibullah. Surmad residents said helicopters had gone into the mountains amid heavy firing Saturday.

The ground troops came under fire as the truck convoys, kicking up a dust storm, approached the mountains from the flat open plains, Raza Khan said.

``You can't do everything in one operation,'' he said Sunday. ``This is Afghanistan. This is a guerrilla war.''

The first stage of the offensive was designed to cut the road from Shah-e-Kot to trap al-Qaida and Taliban forces in the mountains, said Safi Ullah. He said the plan also involves setting up checkpoints in the area to prevent them from escaping.

Pakistan closed its border to prevent escape by any fleeing al-Qaida or Taliban members and deployed extra army units and members of the Khasadar tribal militia to catch any who try to cross the frontier.

The rugged, caved mountains around Gardez have been a hiding place for Afghan warriors since anti-Soviet guerrillas used them as a base for their fight against Soviet troops in the 1980s.

Afghan officials say al-Qaida and Taliban fighters are regrouping in the mountains and just over the border in Pakistan, urging the faithful to wage holy war against U.S. forces.

International aid workers and Afghan sources say al-Qaida and Taliban hiding in the Kharwar district targeted Sunday by airstrikes are being protected by the Taliban's former deputy foreign minister, Abdul Rehman Zahid.

Neither the former Taliban supreme leader Mullah Mohammed Omar nor al-Qaida chief Osama bin Laden is believed to be in the area.

But nearly two months ago, about 250 Taliban and al-Qaida soldiers told residents of Shah-e-Kot village, as well as 20 to 25 other villages tucked inside the mountains and foothills, to leave the area, said Haji Mohammed Gul, a resident of Murgurah-e-Khiel, near Surmad.

Initially, the villagers asked them to go away, he said. ``But the Taliban and al-Qaida, with their Kalashnikovs, told them to leave.''

Hundreds of families who were forced out of their homes now live in Surmad, said resident Gul Akhund, wrapped in a beige wool shawl against the chilly afternoon air.