U.S. and Afghan sources said that anti-Taliban forces took control of the eastern city of Jalalabad and there were further conflicting reports about control of the airport outside the Taliban stronghold of Kandahar. 
Meanwhile, Taliban soldiers at the Pakistan border said the city was still in Taliban hands. 
Neither side's version of events could be independently confirmed.
Still, in neighboring Tajikistan, the Northern Alliance ambassador said the Taliban's power center had fallen to the opposition.
"Today the forces of the Northern Alliance entered Kandahar," ambassador Said Ibrahim Hikmat said.
Meanwhile, Alliance Foreign Minister Abdullah Abdullah earlier told Iran's state television: "There is complete chaos in Kandahar. It's absolute confusion. The Taliban have lost control of the situation and no Taliban officials are to be found."
The various claims could not be independently verified but, if true, they would represent an enormous setback for the Taliban and the al Qaeda network.
In a briefing at the Pentagon, Rear Adm. John Stufflebeem said that anti-Taliban groups in the south were rebelling against Taliban control near Kandahar. However, he said the outcome was not yet clear.
Meanwhile, Pakistani Taliban soldiers, some wounded, trickled across the Pakistan border, saying the fundamentalist Islamic group was still firmly in charge of their southern Afghan stronghold of Kandahar.
"There are about 40,000 or 50,000 Taliban in Kandahar. We are making strong defensive positions," Din Mohammad, bearded and clad in the Taliban's trademark turban, told Reuters at the Chaman border crossing.
Mohammad, who said he was returning to Pakistan to see his family, tell them he was all right and recruit more fighters for the Taliban cause, said the militia was regrouping.
The reported collapse of Kandahar followed the Northern Alliance's seizure of the capital Kabul and the domino-like fall or defection of towns and provinces that reduced the Taliban's control from 90 percent of the country to 20 percent in just five days.
But military analysts expected the Taliban, aided by al Qaeda, to take to Afghanistan's impenetrable mountains in a guerrilla war reminiscent of the hit-and-run campaigns that drove the occupying Soviet army from the country in 1989.
In other battlefield developments, an anti-Taliban group seized control of the eastern city of Jalalabad, an area which housed al Qaeda training camps, Pakistan-based Afghan Islamic Press said.
Four northeastern provinces also slipped from Taliban hands after local uprisings, officials and tribal leaders said.
"Now the Taliban have less than 20 percent of the territory of Afghanistan," opposition Northern Alliance Interior Minister Yunis Qanuni told Reuters.
World leaders were trying to assemble a multinational peacekeeping force and plans for a transitional government for a country racked by civil war since the former Soviet Union invaded on Christmas Day 1979 to back communist rule in the Muslim country.
Diplomats were converging on Afghanistan's southern neighbor, Pakistan, to hammer out post-Taliban rule.
U.N. envoy to Afghanistan Fransesc Vendrell was in the Pakistani capital, Islamabad, waiting to go to Kabul to meet Northern Alliance leaders as soon as U.N. security officials give him clearance that it is safe.
He is expected to push a U.N. plan envisaging a two-year interim government bringing all ethnic groups under one umbrella with a multinational security force to protect them.