KABUL, Afghanistan - U.S. troops battled inside Afghanistan for the first time Saturday, answering a Taliban challenge for a ground campaign. Air strikes also resumed after a lull for the Muslim day of prayer, U.S. officials said.

About 100 American commandos were involved in the fighting which lasted for several hours, said a U.S. official in Washington, speaking on the condition of anonymity. The aircraft carrying the commandos were safely outside Afghan airspace and had returned to base, the official said, providing no details on casualties.

In an indication the ground combat was imminent, U.S. officials said on Friday that special forces were operating in northern and southern Afghanistan in a secretive phase of the war on terrorism, and additional troops are poised for commando raids in search of Osama bin Laden and his al-Qaida lieutenants.

Afghanistan's Taliban regime, meanwhile, welcomed word of the first U.S. special forces troops deploying in its territory, challenging Washington to launch an all-out ground campaign.

"If they want to send in soldiers, they should send in 100,000. Then it can be a fight between our soldiers and theirs," Taliban embassy spokesman Sohail Shaheen said in Islamabad, the capital of neighboring Pakistan.

U.S. jets resumed attacking after midnight, blasting targets northeast of Kabul near dawn Saturday. At least eight large explosions rattled the city.

The renewed attacks from the air and the start of the land action came after a day of relative calm Friday when refugees streamed toward Afghanistan's borders by the thousands to escape Kandahar, the southern city battered by the campaign.

A U.N. official said up to 80 percent of the half-million residents had fled Kandahar, where the country's Taliban rulers have their main headquarters.

In other developments:

_ At the Pentagon, Defense Secretary Donald H. Rumsfeld said for the first time that the U.S. military is coordinating with anti-Taliban forces on the ground, providing food, ammunition and money.

_ A top leader of the opposition forces, Gen. Rashid Dostum, said U.S. personnel have been holding talks with opposition leaders near the northern city of Mazar-e-Sharif. The northern alliance said the Taliban sent reinforcements to try to push back opposition advances on Mazar-e-Sharif over the past two days.

_ Germany issued an international arrest warrant Friday for Zakariya Essabar, 24, a Moroccan suspected of helping plot the Sept. 11 attacks on the United States, saying he was part of the same Hamburg terrorist cell that included three of the suicide hijackers.

_ The News, a Pakistani newspaper, reported that the Pakistan government had given U.S. troops permission to use a third air base _ Dalbandin near the Iranian border _ to support the air campaign against Afghanistan.

_ On the eve of regional economic forum in China, President Bush urged wavering Asian nations to stand up to terrorists. Chinese President Jiang Zemin declared support, but said the war on terrorism should have clearly defined targets and avoid innocent casualties. The president faced resistance from Malaysia, whose leader wants the United States to stop bombing Afghanistan.

Late Thursday and early Friday raids hit Kandahar and around the eastern city of Jalalabad, an area where bin Laden's al-Qaida network has posts.

U.S. jets pounded Kabul before dawn, then returned at midday to drop two more bombs. But the barrage was far lighter than what had been previous days' intensified attacks on Taliban military sites across the capital.

Some Kabul residents used the break in airstrikes Friday to abandon their homes, gathering up belongings and children to find shelter away from what they feared were targets of more likely strikes.

To the south, residents of Kandahar were on the move as well, seeking safety.

Ron Redmond, a spokesman for the U.N. refugee agency, said 3,500 Afghans, most from Kandahar, arrived Friday at the Pakistani border town of Chaman by morning _ nearly three times the number who crossed there over all of the past week.

Some arrived without food or belongings. Afghanistan's neighbors have all closed their borders, fearing such an influx of refugees or militia.

One new arrival, Amir Agha, 42, said thousands more refugees were stranded on the Afghan side of the border _ prevented from getting in by Pakistani border officials who, he said, let through only Afghans with valid documentation.

"We know we will lead a miserable life in Pakistan, in tents," said Abdul Gayyum, who came with his wife and three children. "We have come here just to save our children."

In Islamabad, U.N. spokesman Hassan Fairdous said the attacks had driven out 70 percent to 80 percent of the people of Kandahar, 275 miles south of capital.

Kandahar residents were overwhelming surrounding villages, at times crowding four families into a single mud hut, Fairdous said.

Agha, who arrived at Chaman on Friday, said he had seen bodies lying in Kandahar's streets and that people were dying in hospitals for lack of drugs.

The United States has stressed that its air campaign, launched Oct. 7, is targeting Afghanistan's Taliban regime and bin Laden's al-Qaida terror network, suspected in last month's terror attacks in the United States. Washington says it is trying to minimize harm to civilians.

In Pakistan, Taliban ambassador Abdul Salam Zaeef told reporters the nearly two-week bombing has yet to yield a single casualty among bin Laden's leadership or the Taliban's. Zaeef told reporters he had met with Taliban supreme leader Mullah Mohammed Omar just before returning to Islamabad.

Taliban sources on Friday confirmed a first fatality among bin Laden's close associates in Afghanistan. The Pakistan-based Afghan Islamic Press said Abu Baseer al-Masri was fatally wounded Oct. 11 when a grenade accidentally exploded in his hands. The Taliban denied reports a U.S. bomb had killed al-Masri. Although a veteran member of al-Qaida, al-Masri was not believed to be in the top ranks.

Aid agencies, meanwhile, complained of continued looting of their operations by Taliban and other armed bands. On Friday, the Swedish Committee for Afghanistan said Taliban fighters had sacked offices in five cities, beating some local staff and forcing them to shut down.

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EDITOR'S NOTE _ Kathy Gannon contributed to this report from Islamabad, Pakistan. AP Military Writer Robert Burns contributed from Washington.