WASHINGTON - The next focus of the Afghanistan war - in the south where Osama bin Laden is believed to be hiding in caves - could present the U.S. military with a formidable challenge.

As the Taliban flee from northern cities, American forces could be drawn toward greater use of ground troops or a guerrilla war - even as the ruling militia retreat in an apparent sign of success for President Bush's plan to "smoke them out."

A month of air strikes by U.S. warplanes - including a week of bombing Taliban front lines - has helped rebels take one northern city after another. They marched into the capital, Kabul, yesterday.

"It's the first good news we've had in our war," said Ivo Daalder, defense analyst at Brookings Institution. "But let's not confuse good news with victory."

Indeed, the Pentagon has known for some time that destroying the al-Qaida network and toppling the Taliban who shelter its members might turn more on progress on the southern front - where the Taliban and al-Qaida are strongest.

"We didn't go to war to install the northern alliance in Kabul," said Daalder. "We went to war to get al-Qaida, which is concentrated in the south."

In the advances in the north, Americans contributed weapons, ammunition, supplies and advice but left the ground fighting to a loose coalition known as the northern alliance, which has battled the Taliban for years.

In the south, there is no such rebel force. And Washington has tried with difficulty to find allies willing to start an uprising in the region, which includes Kandahar, the Taliban's headquarters.

Defense Secretary Donald H. Rumsfeld said Tuesday that some U.S. special forces were in southern Afghanistan "doing things that are helpful to our side and unhelpful to the other side." But, unlike the American forces in the north, those in the south are not working with any anti-Taliban groups, Rumsfeld said.

U.S military planners think the best course is to approach ethnic Pashtun tribal leaders in the south who are unhappy with the Taliban - and persuade them to defect. The CIA has been out in front on the effort, trying to identify such individuals or groups for the Pentagon to equip and arm, officials have said.

Assembling an opposition force from among the Pashtuns has been hard. They are the country's largest ethnic group and the south's largest - and they make up the backbone of the Taliban.

That's another difference from the north, which is mostly ethnic Tajiks and Uzbeks, groups that dominate the northern alliance.

Before the alliance began gaining ground in the north, there hadn't been enough reason for southern commanders or other leaders "to stick their necks out" and go against the Taliban, said Michael Vickers, a former Green Beret and CIA officer.

"Whether this psychologically unhinges" their loyalty to the Taliban "and emboldens them remains to be seen," he said.

Without defections to form a strong fighting force in the south, the Pentagon might have to put more of its own troops on the ground to do more of the fighting, analysts said.

Rumsfeld said the United States would pursue the Taliban and al-Qaida wherever they go.

"If they reorganize in the south, we're going to go get them. If they go to ground, we will, as the president said, root them out," Rumsfeld said at a Pentagon news briefing. "And if they decide to flee (Afghanistan), I doubt that they'll find peace wherever they select."

There were signs yesterday the Taliban were abandoning Kandahar and other urban centers in the south, possibly to wage a guerrilla war from the mountains.

"That's a possibility and even likely," said retired Air Force Gen. Merrill McPeak. "But opposition forces would be in control of towns and crossroads and will be in a position to end support for al-Qaida, which is exactly what we've been after."

Washington has been hoping that successes in the north would help persuade Pashtun leaders in the south to turn against the Taliban. Analysts said it's possible the alliance's march through so much of the north in just a few days may have exactly that effect.

"The Taliban are really on the ropes," said Vickers. "It's important that this momentum be exploited."