At the Pentagon, Rear Adm. Craig Quigley, a spokesman for Defense Secretary Donald H. Rumsfeld, said the reported breakthrough would be a welcome development if true.
Rashid Dostum, the Uzbek warlord who controlled Mazar-e-Sharif until the Taliban captured the city three years ago, said in Turkey that the alliance overran the city in a half hour.
He said he was speaking by satellite telephone from a hill overlooking Mazar-e-Sharif. Dostum claimed northern alliance forces killed 500 Taliban fighters and took hundreds of others prisoner during the past four days of fighting.
Dostum claimed the alliance suffered 28 killed and more than 30 wounded. There was no immediate Taliban comment on the reports the city has been taken.
In the past few days, opposition forces have credited intense American bombing against Taliban front lines with helping their advance on Mazar-e-Sharif. U.S. warplanes struck Taliban positions around the city Friday as well as on the other main front, north of Kabul.
Several explosions resounded on the outskirts of Kabul, and huge plumes of smoke rose along the front line about 30 miles north of the city. Taliban soldiers fought back with anti-aircraft guns after several days of holding their fire.
Col. Zia Hauddin, an opposition commander, said the Taliban had reinforced the front line with about 2,000 troops, mostly Arab and Pakistani volunteers. The Taliban have also brought in ammunition, tanks and other vehicles, he said.
The U.S. bombing "should be accelerated," he said.
Earlier Friday, fighting was heavy between opposition and Taliban forces around Mazar-e-Sharif, with the two sides giving conflicting accounts. The Pakistan-based Afghan Islamic Press quoted an unidentified Taliban spokesman as saying that troops repulsed two opposition attacks on the city Friday.
Alliance officials said they are counting on wholesale defections among Taliban forces in and around Mazar-e-Sharif and an uprising by the city's residents to avoid bloody house-to-house fighting. Taliban commanders, however, have said the morale of their troops is high.
Elsewhere, U.S. jets and B-52 bombers repeatedly hit Taliban targets overnight and early Friday north of Kabul and around Kandahar, the southern city that is the Taliban headquarters.
Witnesses reported about 30 bombs near Bagram, the site of an air base north of Kabul that is controlled by anti-Taliban forces. The opposition has not been able to use the airfield because of the proximity of Taliban troops.
The Taliban fired anti-aircraft guns at U.S. planes and shelled opposition forces, witnesses said. Taliban fire had diminished recently, though it was not clear whether the militia was conserving ammunition or had lost guns to the bombing.
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