KABUL, Afghanistan - Taliban fighters were fleeing positions north of the capital yesterday as truckloads of opposition troops advanced, shouting "God is great." Opposition fighters also seized western Afghanistan's biggest city, the opposition said.

Northern alliance fighters waved their green-and-white flags and plastered pictures of slain military leader Ahmed Shah Massood on their trucks as they shored up gains in the first significant advance by the opposition on the front north of Kabul.

The Taliban deployed tanks at major roads leading into the capital in anticipation of an all-out assault. Security was dramatically increased, with nervous, heavily armed Taliban fighters searching vehicles at key intersections.

A senior opposition spokesman, Bismillah Khan, said anti-Taliban forces were "at the gate of Kabul."

Speaking by satellite phone, he said opposition troops had moved into Mir Bacha Kot, about 12 miles north of the capital. The route from there into Kabul is a broad, rocky, barren plain flanked by mountains.

The opposition government's foreign minister, Abdullah Abdullah, said some fighters had advanced as far as Shakar Dara, a few miles closer to Kabul.

President Bush has urged the opposition to hold off on seizing the capital until a broad-based government can be formed to replace the Taliban, the ruling Islamic militia. While some opposition leaders - including Abdullah - say they agree, some commanders on the ground were eager to advance.

It was unclear whether the opposition had gained so much momentum that an assault on Kabul was inevitable. In three days, the opposition has expanded its control from about 10 percent of the country to nearly half.

U.S. jets prowled the skies over Kabul, most headed toward the front line. Yesterday afternoon, a missile slammed into the Wazir Akbar Khan neighborhood, setting off a huge explosion.

The neighborhood is home to several prominent Taliban officials as well as Arabs, Chechens and Uzbeks believed linked to terrorist suspect Osama bin Laden.

The action north of Kabul came as opposition fighters claimed to have entered Herat, the main city in western Afghanistan, and to be closing in on the last Taliban stronghold in the north.

Opposition spokesman Mohammed Abil said the opposition entered Herat in the morning. Tehran TV, reporting from Herat, said the opposition was in full control of the city.

An official in the Taliban's Information Ministry said "possibly Herat has collapsed."

Herat sits along the main road to Kandahar - more than 300 miles to the southeast - which is the birthplace of the Taliban and home of Taliban supreme leader Mullah Mohammed Omar.

Taliban control across the north of the country has collapsed since the fall of Mazar-e-Sharif to the opposition on Friday.

In Mazar-e-Sharif, men lined up at barber shops to have their Taliban-mandated beards shaved off. Women were discarding the all-encompassing burqas, and music - banned by the Taliban - could be heard coming from shops, according to the Afghan Islamic Press.

But U.N. officials said the city was still volatile, with reports of abductions of civilians and summary executions. Opposition fighters seized a U.N. aid convoy Saturday, and a U.N. food warehouse had been looted - though it was not known by whom, U.N. officials said.

U.N. spokeswoman Stephanie Bunker, speaking in Islamabad, Pakistan, said the reports of executions were still "unconfirmed" and had few details. It was not known if the killings involved alliance fighters taking revenge against Taliban or pro-Taliban residents - something opposition commanders had said they would try to prevent.

Abil, speaking by satellite telephone, said alliance forces were preparing to move against Kunduz, the last northern city still held by the Taliban. The area is populated mostly by ethnic Pashtuns - the same ethnic group as the Taliban - while the rest of the north is largely Tajik, Uzbek and Shiite Muslim.

Late yesterday, Tehran radio reported that opposition forces had taken control of Kunduz. Opposition spokesmen contacted by satellite telephone said they did not have updated reports on the operation.

In Kabul, meanwhile, Taliban judges indefinitely postponed the trial of eight foreign aid workers, saying they feared their anger over the U.S. air strikes would prevent them from making a fair ruling. The defendants - two American women, two Australians and four Germans - are accused of spreading Christianity in Muslim Afghanistan.

Developments on the battlefield were so fast-moving that many of the reports could not be immediately verified. Foreign journalists do not have access to many of the front lines and have been speaking to opposition commanders by satellite phone.

On the front north of Kabul, trucks of jubilant fighters rumbled through Rabat, a town along the route of advance. Asked where the trucks were going, one opposition soldier, Commander Adel, shouted "to Kabul, to Kabul."

The fighters had pictures of Massood, the alliance's military chief, who was killed by a suicide attack just before the Sept. 11 terror attacks in the United States.

The speed of the Taliban collapse suggests that many local commanders and Taliban fighters are switching sides rather than offering stiff resistance.

Abil said the northern alliance has sent radio messages to Taliban commanders and village elders urging them to hand over Pakistani, Arab and Chechen volunteers fighting with the Islamic militia.

"We want to take these foreigners alive to show who is fighting against us," he said. He claimed the greatest resistance was coming from the foreign fighters.

It remained unclear whether the opposition could maintain their momentum as they approach Taliban strongholds in the southern Pashtun heartland.

In Islamabad, the Taliban's ambassador in Pakistan, Abdul Salam Zaeef, acknowledged that the Islamic militia had withdrawn from seven northern provinces.

"The Islamic army of the Taliban withdrew from these provinces in an organized way to avoid civilian casualties," he said in Pakistan's capital Islamabad.

Washington wants the opposition to hold off on assaulting Kabul to avoid a repeat of factional fighting that destroyed the capital and killed an estimated 50,000 people from 1992 to 1996, when the opposition governed.

Two French radio journalists and a German magazine reporter were killed Sunday when their convoy was hit in northeastern Takhar province, Radio France Internationale and RTL radio announced in Paris.