The United States has defeated the Taliban in Afghanistan, but Al Qaeda terrorists are still at large and will be difficult to find, a top Pentagon official said Monday.

"It ain't over yet," Deputy Defense Secretary Paul Wolfowitz told reporters at a Pentagon briefing. "We've accomplished our one major objective, which is the defeat of the Taliban. ... Large numbers of Al Qaeda terrorists are still at large. It's going to be a very long and difficult job."

As U.S. bombers continued to rain weaponry on suspected terrorist hide-outs in the mountains of Afghanistan, Wolfowitz said it is very difficult to determine how many terrorists, their supporters, or even key leaders have been killed in the operation.

Wolfowitz said there is no clear evidence that Usama bin Laden has been able to flee Afghanistan, but the war against his network has seriously impeded his ability to operate, Wolfowitz added.

"This is a man on the run," the deputy secretary said. "He doesn't have a lot of good options."

The U.S. effort will continue, Wolfowitz said. U.S. forces will continue to focus on blocking possible escape routes and to hit sanctuaries where the terrorists might have fled.

Earlier, Pentagon spokeswoman Victoria Clarke said the fighting in Afghanistan is fierce. "It's pretty intense fighting and pretty intense activity," she said.

Speaking of U.S. bombing and anti-Taliban fighting in the holdout areas around Jalalabad and Tora Bora, Clarke said, "There are a lot of reports (of) a fair amount of action up there. Fighting remains fierce. ... It's tough."

On Sunday, military officials said U.S. soldiers probably would be in Afghanistan for a long time, despite the Taliban's collapse and opposition forces surrounding an Al Qaeda hideout. More American forces could be sent to Afghanistan to help in the hunt for bin Laden, other Al Qaeda leaders and officials of the disintegrating Taliban.

"We may send in some," Wolfowitz said Sunday. "The most important thing for the American people to understand is our objectives remain very largely to be done in the future. Enemies that are half-defeated can be very dangerous and they can take a long time to clear out."

U.S. forces would not occupy Afghanistan, but would hunt down top Taliban and Al Qaeda leaders, continue humanitarian aid efforts and help support a post-Taliban government, the officials said on Sunday television news shows.

"We're not eager to have the United States come in and become an occupying power in Afghanistan. That's not our purpose," Vice President Dick Cheney said.

"We want to see to it that what is left behind gives the Afghan people the opportunity to develop a strong representative government, a government that can guarantee that, in the future, no terrorist will once again find sanctuary or safe harbor in Afghanistan," Cheney said on NBC's Meet the Press.

That goal, he said, could take years of involvement and would rely on aid agencies and perhaps U.N. peacekeepers over the short term.

U.S. troops are unlikely to remain in Afghanistan to provide security for food and other humanitarian aid distribution, added Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff.

"This global war on terrorism is going to require military forces for some time to come, perhaps, and one thing we don't want to do is leave a large legacy force (of anti-U.S. fighters) in Afghanistan," Myers said.

Military officials believe bin Laden probably is in the Tora Bora area, a complex of caves and tunnels in mountainous eastern Afghanistan.

U.S. special forces troops with the anti-Taliban forces there "are trying to get their eyes onto some of these targets" to help with airstrikes and aid the opposition, Myers said. The fighting in the area is fierce, said Myers.

"The Al Qaeda forces that ... in some respects are trapped up there are fighting for their lives," Myers said.

Pentagon officials believe Mullah Mohammed Omar is probably in the Kandahar area after fleeing the city, Wolfowitz said. The United States plans to hunt down Omar and the remaining Taliban leaders, either with or without Afghans' help, Myers said.

In London, British Defense Secretary Geoff Hoon said that if his country's troops captured bin Laden, they would hand him over to the United States only on the condition he would not face the death penalty. Myers said he had not heard that was the British position.