BANGI, Afghanistan, Thursday, Nov. 15 — A large garrison of Taliban troops trapped in Kunduz, in northern Afghanistan, continued to defy the Northern Alliance on Wednesday as warlords from both sides met for several hours to try to avert a bloodletting.

Commanders from the Taliban and the Northern Alliance gathered in Emam Sahib, about 30 miles north of here, in an effort to negotiate a peaceful handover of the city.

By early today, it appeared negotiations had broken down, as American B-52's and fighter-bombers began striking Taliban positions outside Kunduz.

At the same time the commanders were meeting, the Northern Alliance beefed up its forces that have encircled Kunduz, adding some 2,000 troops and a dozen tanks in the last 24 hours.

Northern Alliance leaders say Kunduz holds as many as 20,000 Taliban and foreign fighters, who they say fled there after defeats in nearby Mazar-i-Sharif, Taliqan and elsewhere. Those estimates were impossible to verify, but the city appears to contain the largest concentration of Taliban troops left in northern Afghanistan.

As Taliban forces disintegrate or flee toward Kandahar, in the south, the garrison in Kunduz has stayed put. Northern Alliance troops, who moved to within 20 miles of the city earlier this week, say they have blocked all roads leading out.

With thousands of troops dug in on each side, the showdown could be one of the last major confrontations between the Northern Alliance and the remnants of the Taliban.

Northern Alliance warlords said most of the Taliban commanders who control Kunduz had agreed to surrender the city and switch sides. The sticking point, they said, is the fate of the thousands of foreign fighters — mostly Arabs and Pakistanis — and hardcore Taliban "terrorists" they say are holed up there.

Northern Alliance leaders say they want to arrest, jail and possibly execute many of the foreign and extremist Taliban fighters, who they say have committed atrocities against the Afghan people. They said that they would attack Kunduz today if the Taliban did not agree to give them up.

"These foreigners have killed thousands of civilians," said Pir Muhammad, a senior Nothern Alliance commander. "Their hands are covered with the blood of our people. We will avenge this."

The negotiations at Emam Sahib followed a humiliating retreat by the Northern Alliance on Tuesday. In that episode, alliance troops who had been invited into Kunduz by a Taliban commander walked into an ambush. The Taliban fighters fired rockets, prompting a chaotic retreat by more than 1,000 alliance fighters.

The negotiations appeared labyrinthine and tense. Alliance leaders said they were negotiating simultaneously with as many as 20 local Taliban commanders inside Kunduz by radio, trying to persuade each of them to defect. Taliban officials agreed to the talks at Emam Sahib only when the alliance guaranteed them safe passage.

"We have been talking to them day and night," said Rosmuhammad Uria, a Northern Alliance commander. "Sometimes they are with us, sometimes not."

The alliance's dash across Afghanistan in recent days was made possible as much by Taliban defections as battlefield prowess. The alliance secured the defection of some 5,000 Taliban troops in Taliqan earlier this week, allowing the Northern Alliance to take the city without a battle.

The situation in Kunduz appears different, in part because of the large number of foreign fighters who appear to be ensconced there and also because here, unlike other sites of Taliban defeats, the losers have nowhere to run. Though Kunduz has a large population of the Pashtun ethnic group, who make up a majority of the Taliban, most of the people of northern Afghanistan belong to other groups.

Alliance commanders said they were prepared to spare most of the Taliban soldiers now in Kunduz but wanted to detain some 3,000 foreign troops who they said were there. The commanders declined to say how many Taliban "terrorists" of Afghan origin they want to arrest in Kunduz, but they said the number was substantial.

Concerns about reprisals against the Taliban have grown. In Mazar-i- Sharif, the United Nations said, there were credible allegations that more 100 young Taliban recruits who had been taken prisoner had been executed. In Taliqan, Northern Alliance officials said, they have arrested 400 members of the Taliban but they have given out no details since.

Pir Muhammad, the Northern Alliance leader, said he would have little trouble picking out the "terrorists" and foreigners from among the thousands of other troops in Kunduz. He said Northern Alliance spies in Kunduz had already identified the suspects.

"We know who the criminals are," he said.

Some Northern Alliance leaders said they feared that a core of Taliban extremists might be blocking an agreement to hand over Kunduz, and that the various factions could start fighting each other inside the city.

Along the front lines in Bangi, many of the soldiers said they expected the Northern Alliance to reach an agreement with some members of the Taliban but not with others.

"Some we will embrace; some we will fight,"said Sher Aga, a soldier on the front lines here.

With the two armies facing each across the barren mountains that separate Kunduz and Takhar Provinces, rumors of foreign involvement circulated as well.

Northern Alliance commanders said that a pair of Pakistani Air Force planes had landed on Tuesday at the airport in Kunduz , apparently with the permission of the American government, to evacuate a number of Pakistani and Taliban officials.

Abdul Malik, a smuggler who crossed the front lines from Kunduz, said he had heard two propeller airplanes land at Kunduz on Tuesday. Such a development would be unusual, because the Taliban air force was destroyed during the American bombing campaign and American planes control the skies.

The accounts could not be confirmed, but Sher Ahmed, a Northern Alliance commander in Taliqan, said that on Monday, Taliban leaders had tried to send a helicopter to Kunduz but that it had been intercepted by an American military helicopter. His account could not be confirmed.