After days of seesaw fighting, the opposition Northern Alliance said today that it had captured the strategic northern Afghan city of Mazar-i-Sharif.

The Pentagon said the report could not immediately be confirmed. But if it is verified, the alliance's move would be the most significant gain against the Taliban since airstrikes on Afghanistan began on Oct. 7, because it would help to establish a land bridge to Uzbekistan for bringing in military supplies and relief aid.

"We have gotten bits and piece of information, but I can't confirm that they have entered the city," the Pentagon spokeswoman, Victoria Clarke, told Reuters.

An opposition spokesman said the Taliban appeared to have abandoned the city, but the Pentagon continued its cautious approach to the claims at a press briefing today.

At the briefing, Ms. Clarke was reluctant to add much to her earlier statement.

"It's a fluid situation and we just couldn't give you an accurate picture right now," she said. "What we have seen is encouraging. We are not going to say more than that."

She stressed that "beyond that we not going to characterize it, we are not going to reveal every detail of the pieces of information that we have."

Rear. Adm. John D. Stufflebeem agreed with Ms. Clarke's assessment of the situation and said: "There's a lot of dust in the air right now. There are skirmishes happening across these various fronts, if you want to call them as such, and with that dust in the air it's very hard to tell how exactly what's going on."

The admiral said the Pentagon was not reluctant to confirm the Northern Alliance claims because of a belief that the opposition forces might not be able to hold on to any territory it had taken over. It was more a question of "Where are we now right?" and that as battles were apparently still going on, "it's hard to tell what is the likely outcome," he said.

Asked how the Pentagon would confirm the claims, Admiral Stufflebeem, a deputy director for the Joint Chiefs of Staff, said: "We don't know enough yet to know what's been taken, will it be held and then, therefore, will there be more movement from that? So I think we just need some perspective to see how that will play out."

The United States has supported the alliance battle around Mazar-i-Sharif for the last four days by persistent bombing of Taliban forces and supplying military and other supplies. Americans have also been reported on the ground helping with targeting and directing alliance attacks on villages around the city.

"The alliance forces today took over the airport and entered the city," the group's spokesman, Ashraf Nadeem, told Reuters by satellite telephone.

In a similar interview with The Associated Press, Mr. Nadeem said the Taliban appeared to have abandoned the city. He said opposition forces had broken through Taliban defenses at the Pul-i-Imam Bukhri bridge on the southern edge of the city.

"We are moving through one neighborhood at a time," he said.

Gen. Abdul Rashid Dostum, the Uzbek warlord who controlled Mazar-i-Sharif until the Taliban captured the city three years ago, told CNN-Turk television of Turkey that the alliance overran the city in a half hour.

The general, who said he was speaking by satellite telephone from a hill overlooking the city, claimed that Northern Alliance forces had killed 500 Taliban fighters and took hundreds of others prisoner during the last four days of fighting.

General Dostum asserted that 28 fighters from the alliance had been killed and more than 30 wounded. There was no way to confirm the casualty claims, which both sides have exaggerated in the conflict.

Taliban troops appeared to be retreating east toward Samangan Province, General Dostum said. There was no immediate comment from the Taliban.

The fall of Mazar-i-Sharif would be a big blow for the Taliban. The city straddles crucial supply routes between Kabul, the Afghan capital, and Uzbekistan, and also offers a working airport.

Control of the city would give the United States and its allies the ability to bring in large quantities of ammunition, weapons and other supplies to aid the alliance. It could also be used to bring relief aid to Afghanistan.

In another development, hundreds of troops of opposition forces, backed by tanks, were reported massed at the front line just north of Kabul late today local time, and commanders said they expected to advance within hours.

A Reuters cameraman saw about 800 Northern Alliance fighters march toward the front line along with four tanks and 40 artillery pieces while United States jets roared overhead, dropping bombs on Taliban positions overlooking the opposition-held Bagram airport.

Several explosions resounded on the outskirts of the capital, and huge plumes of smoke rose along the front line about 30 miles north of the city. Taliban soldiers fought back with antiaircraft guns after several days of holding their fire.

Admiral Stufflebeem said he not seen reports of the massing of troops.