The report -- which, if confirmed, would represent a dramatic new development in a campaign the United States has so far mainly conducted from the air -- came at the end of another bloody day in Afghanistan, with new setbacks for the Taliban.

U.S. warplanes and helicopters rushed into action to quell a revolt by several hundred prisoners loyal to Osama bin Laden in the headquarters of the Northern Alliance, a key U.S. ally.

Hundreds of people were reported dead or wounded in the battle, which went on for several hours before the uprising was crushed and the Northern Alliance re-established control.

Elsewhere in Afghanistan, resistance in the Taliban stronghold of Kunduz finally crumbled, and Taliban forces also began to withdraw from the town of Spin Boldak, near the Pakistan border.

President Bush vowed to topple the Taliban after they refused to hand over bin Laden, who the United States says masterminded the Sept. 11 attacks on New York and Washington that killed more than 3,900 people.

The Taliban is now essentially confined to Kandahar, where its rigid brand of Islamic fundamentalism first took root. But the final assault on the city appeared to be close.

Anti-Taliban tribes took control of the airport near Kandahar on Sunday. Shortly afterward, one of their spokesmen reported U.S. ground troops had begun landing by helicopter.

Mohammad Anwar, spokesman for Gud Fida Mohammad, a commander of the Achakzai tribe, said huge aircraft were circling overhead while a stream of helicopters flew constantly in and out of the airfield.

Some of the helicopters were described as Chinooks bringing in armored vehicles. This would be the first such U.S. armor to land in Afghanistan since the United States launched its attacks on Oct. 7.

But the reports could not be independently confirmed, and the Pentagon declined to comment. "We cannot discuss future or ongoing military operations or troop movements," said Air Force Lt. Col. Ken McClellan, a Pentagon spokesman.

BATTLE IN MAZAR-I-SHARIF

The Northern Alliance and other anti-Taliban fighters have been taking thousands of prisoners, both Afghans and foreigners from the Arab world, Pakistan, Chechnya and elsewhere who had streamed to Afghanistan to fight under bin Laden's banner of holy war.

Amid reports of prisoner massacres, many of the captives are said to be desperate. The Pentagon said Sunday's prisoner revolt erupted after about 300 fighters identified as non-Afghan Taliban wielding smuggled weapons started a firefight in the headquarters of Gen. Abdul Rashid Dostum near the city of Mazar-i-Sharif.

The Northern Alliance commander, who captured the town earlier this month, mustered about 500 of his troops to counterattack, "and we provided support via airstrikes," said Army Lt. Col. Dan Stoneking, another Pentagon spokesman.

A reporter for Time magazine said he thought a U.S. serviceman had been killed, but the Pentagon said it knew of no U.S. military casualties.

One defense official, who declined to be identified, said the non-Afghan Taliban fighters had held the southern part of the complex before U.S. AC-130 gunships and Black Hawk helicopters helped the Northern Alliance restore control.

A Reuters correspondent saw U.S. fighter jets sweep over and drop at least four bombs on the southern part of the fort, where the insurgent foreign prisoners were concentrated.

REPORTERS CLIMB DOWN WALLS

Journalists, including two from Reuters, Red Cross officials and two U.S. observers were trapped inside the fort for hours. Most escaped by climbing down the fort's 65-foot outer wall amid gunfire.

Around Kunduz, Northern Alliance officials said rival forces led by the ethnic Uzbek Dostum and ethnic Tajik commander Mohammad Daoud advanced toward the city, apparently intent on entering it on Monday.

Daoud said the town of Khanabad, the eastern gateway to Kunduz, had fallen and forces under his command were racing to the city, 12 miles away. He said he hoped his forces would enter Kunduz en masse on Monday.

"We plan to enter Kunduz city tomorrow," the commander told Reuters at his mud-walled bunker east of Khanabad. "We want to avoid fighting, and we are still negotiating with the Taliban in Kunduz and hope to capture it without a fight."

Preparations for U.N.-sponsored talks in Germany this week on a new government for Afghanistan accelerated as nominal Northern Alliance leader Burhanuddin Rabbani said moderate former Taliban could be included in a future government.

In Kunduz, thousands of the estimated 15,000 Taliban defenders had already surrendered after a 10-day siege by Alliance troops backed by U.S. bombing.

Commander Ustad Attah, allied to Dostum, who retook Mazar-i-Sharif from the Taliban two weeks ago and whose forces were besieging Kunduz from the west, said it now looked as if the city would not be entirely taken until early on Monday.

ETHNIC RIVALRY SPLITS FORCES

Days of confusion over the surrender have been attributed to rivalry between the Uzbek and Tajik ethnic factions of the Northern Alliance, which both want to win control of the city.

Daoud told reporters that Juma Namangani, an Islamic militant leader from Uzbekistan who was a longtime ally of bin Laden, had been killed while defending Mazar-i-Sharif with the Taliban.

Namangani had spearheaded a militant campaign to establish Islamic rule in Uzbekistan and been sentenced to death there.

Regional leaders from Afghanistan's biggest ethnic group, the Pashtuns, who made up the core of the Taliban, said after a meeting in the Pakistani town of Quetta that the Taliban must now surrender Kandahar peacefully.

Pakistan in particular has expressed fears that the Pashtuns, who make up 40 percent of the Afghan population, may find themselves without a voice if the Taliban are entirely shut out of efforts to form a new government.

The Northern Alliance has made a significant political gain by recruiting Mullah Khaksar, a former Taliban deputy interior minister, who made his first public appearance on Saturday since defecting when Kabul fell on Nov. 13.

Switching sides has a long tradition in Afghanistan. At a news conference, Khaksar asked all ethnic groups and tribal chiefs to take part in the search for a broad government.

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