WASHINGTON -- Senior U.S. intelligence and defense officials, caught off guard by the rapid collapse of Taliban control over much of Afghanistan, fear that Osama bin Laden may try to flee the country before he can be captured or killed.

The CIA believes Bin Laden is still on the run in the rugged reaches of southern Afghanistan, but officials have begun sketching out escape routes he might use to try to elude U.S. Special Forces, British commandos and anti-Taliban Afghan fighters.

Officials said Bin Laden may try to use a helicopter to reach another aircraft waiting to take him out of the country. Or he may try to travel overland in disguise through Turkmenistan and southern Russia to join Muslim separatists in Chechnya. The most likely refuge, they said, is across the border in the desolate tribal areas of northwest Pakistan.

But little is certain in this unpredictable war turned unconventional manhunt. Defense Secretary Donald H. Rumsfeld said Thursday that Bin Laden could well evade the search by elite military units and surveillance systems, including imaging satellites and unmanned spy planes.

"Anything is possible in that country," Rumsfeld said. "You've got porous borders in a number of directions. . . . It's not possible electronically to detect everything at all times."

He said it is possible that a helicopter could avoid detection by flying low through deep ravines. Bin Laden also could ride "on a donkey or a burro, a mule, a horse, a truck. . . . I think we'll find him either there or in some other country," Rumsfeld said. "But one has to be realistic."

A congressional aide familiar with current intelligence said Bin Laden is unlikely to escape by air, however. "We know just about everything moving there and, if it's moving, we're going to take it down," he said. "He's got to get out via a land route, and Pakistan is a logical exit."

Bin Laden Prefers Death Over Capture

If Bin Laden is cornered, intelligence officials have told members of Congress, he has made his wishes clear to his closest aides in the past.

"Bin Laden has instructed his sons and other close followers to kill him if it appears he's going to be captured," said Rep. Nancy Pelosi (D-San Francisco), ranking Democrat on the House Select Committee on Intelligence.

Although President Bush announced earlier this week that foreign terrorists may be brought to trial before closed-door military tribunals, Pelosi said it is probably pointless to plan for a military trial of Bin Laden.

"That's not going to happen," she said. "It's not even worth discussing."

U.S. intelligence officials said they believe the suspected terrorist mastermind has reduced the entourage of several dozen lieutenants, bodyguards and others who usually travel with him in a heavily armed convoy of all-terrain vehicles.

"It's partly guess and partly analysis that he's trying to reduce attention to himself and lessen the likelihood that he'd be caught at a roadblock, or that someone would take a picture from the sky," said a senior intelligence official.

The official, who spoke on condition of anonymity, said electronic intercepts, reports from refugees and other intelligence indicates that Bin Laden has "wisely attempted to avoid any pattern at all" in his movements.

"He has been very conscious of his personal security in terms of what time he moves, how long he stays and where he goes," the official said. "He's done all the right things to save himself. He's no dummy."

As evidence, he added, "Bin Laden has cultivated the image that he lives out of caves and tents, rifle at the ready. We think most of the time he's had a roof over his head as opposed to rock."

The official said CIA doctors who have studied Bin Laden's recent videotaped statements discount published reports that he was treated recently for a severe kidney ailment.

"We don't have a basis right now for believing he's sick or has a physical ailment," he said.

The official said the CIA had not foreseen the sudden collapse of Taliban control over much of Afghanistan and is still playing catch-up. Moreover, he said, there hasn't been much useful intelligence from the ground in Afghanistan.

"At this particular moment, the amount of information that is being compiled in classified channels by government entities is a tiny increment above what we're seeing and reading openly" in the press, he said. "It's such a fast-moving situation that no one can keep up."

U.S. Special Forces teams have secured landing strips and set up roadblocks in southern Afghanistan in hopes of interdicting fleeing Taliban troops or Al Qaeda members.

But so far, another intelligence official said, no intelligence bonanza has emerged from interrogation of defecting and captured Taliban troops, or from the seizure of records and documents.

"There are some people who are in the process of negotiating surrenders who are more than your run-of-the-mill Taliban guys," he said. However, he added, only a limited number of people probably have a good idea where Bin Laden really is.

The CIA still doesn't know who was inside a building used by senior Al Qaeda members that was flattened by a U.S. airstrike Tuesday, the officials said.

"We just had intelligence that indicated that there was a potential gathering of Al Qaeda folks," one official said. "So it was whacked."

A senior member of the anti-Taliban Northern Alliance told reporters last week that Bin Laden was constantly moving among three contiguous southern provinces--Helmand, Kandahar and Oruzgan. The last is the largest and most mountainous, and thus the most likely hide-out.

A senior Pakistani intelligence official said Islamabad doesn't have a clue where Bin Laden is hiding.

"We don't know, and we are afraid to know," the official joked, reflecting Pakistan's fear that Bin Laden might follow fleeing Taliban troops and cross into one of the tribal areas where the government has little control.

Armed Spy Planes Join the Hunt

Pakistan's army has sent reinforcements into the Northwest Frontier province in the last two days to try to prevent the Taliban from using the area as a refuge and military staging area.

U.S. officials said they hope the increasing military pressure on the Taliban and Al Qaeda will flush Bin Laden into the open, or catch him using a satellite telephone or other communications device that can be traced and intercepted. In some cases, a communications signal can be used as a kind of beacon to help target missiles.

The CIA has deployed unmanned Predator spy planes armed with Hellfire missiles. The spy planes can provide infrared video at night, intercept communications and provide other key intelligence.

U.S. Special Forces and intelligence operatives have offered rewards of up to $25 million and shown pictures of Bin Laden to local Pushtun tribesmen who may have clues to his whereabouts.

A former CIA officer, speaking on condition of anonymity, said the chase may end with no body and no clear evidence that Bin Laden was killed.

"But the guy won't appear on any more videotapes," the former officer said. "That will be the definitive proof."

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Times staff writers Paul Watson in Kabul and Rone Tempest in Islamabad contributed to this report.