Northern Alliance fighters have entered Kandahar province, the Pentagon said yesterday.
Amid reports of an alliance push into the southern Pashtun-dominated region, US officials suggested the city of Kandahar, the main Taliban stronghold, remained under Taliban control.
But Rear Admiral John Stufflebeem, deputy director of operations for the joint chiefs of staff, acknowledged that an advance on the city by the alliance would be "a problem that we will all have to work through".
He said it remained surrounded by opposition fighters dominated by the region's majority Pashtun ethnic group.
The Pashtun groups are strongly opposed to the mainly Tajik and Uzbek fighters of the Northern Alliance taking control of southern Afghanistan.
Taliban commanders in the city appeared to be hardening their resolve yesterday after broadcasts by Mullah Omar, the movement's leader, urging them to fight "to the last breath".
His message followed the massacre earlier this week by Northern Alliance forces of several hundred Taliban prisoners in the northern town of Mazar-e-Sharif and claims denied by the Pentagon that 160 captured Taliban soldiers were shot by Pashtun opposition forces near Kandahar.
Reports from Afghanis crossing the border into Pakistan suggest foreign volunteers, mostly Arab and Pakistani, have threatened Taliban defectors with death.
Yesterday, Taliban authorities hung a suspected pro-US spy in Kandahar having allegedly found the man with satellite equipment, according to eyewitnesses. The body was left hanging all day in a public square.
Many Taliban have already defected to opposition groups or deserted their posts. But up to 20,000 are believed to remain in position in Kandahar. "It seems as if the core foreign Taliban volunteers are digging in for a fight," said a Pashtun commander based in Quetta, Pakistan, near the border with Kandahar province.
Around 1,000 US marines are now based at an airfield close to Kandahar, where they have been readying a forward operating base as the US hunts for Taliban and al-Qaeda terrorist leaders.
In the town of Spin Boldak, near the border with Pakistan, opposition groups yesterday continued to negotiate with Taliban commanders for their surrender. Earlier this week, opposition forces claimed to have captured the town.
According to witnesses, Arab Taliban fighters in Spin Boldak are resisting surrender. "There are still Taliban everywhere," said a spokesman in Quetta.