The intelligence service even used Al Qaeda camps in Afghanistan to train covert operatives for use in a war of terror against India, the Americans say.

The intelligence service, known as Inter-Services Intelligence, or I.S.I., also maintained direct links to guerrillas fighting in the disputed territory of Kashmir on Pakistan's border with India, the officials said.

American fears over the agency's dealings with Kashmiri militant groups and with the Taliban government of Afghanistan became so great last year that the Secret Service adamantly opposed a planned trip by President Clinton to Pakistan out of concern for his safety, former senior American officials said.

The fear was that Pakistani security forces were so badly penetrated by terrorists that extremist groups, possibly including Mr. bin Laden's network, Al Qaeda, would learn of the president's travel route from sympathizers within the I.S.I. and try to shoot down his plane.

Mr. Clinton overruled the Secret Service and went ahead with the trip, prompting his security detail to take extraordinary precautions. An empty Air Force One was flown into the country, and the president made the trip in a small unmarked plane. Later, his motorcade stopped under an overpass and Mr. Clinton changed cars, the former officials said.

The Kashmiri fighters, labeled a terrorist group by the State Department, are part of Pakistan's continuing efforts to put pressure on India in the Kashmir conflict. The I.S.I.'s reliance on Mr. bin Laden's camps for training came to light in August 1998, when the United States launched a cruise missile attack against Al Qaeda terrorist camps near Khost, Afghanistan, in response to the bombings of two American Embassies in East Africa. The casualties included several members of a Kashmiri militant group supported by Pakistan who were believed to be training in the Qaeda camps, American officials said.

Since the attacks on the World Trade Center and the Pentagon on Sept. 11, the Pakistani government, led by Gen. Pervez Musharraf, has turned against the Taliban and Al Qaeda in favor of the United States.

One element in that shift was General Musharraf's decision to oust the chief of the intelligence service, Lt. Gen. Mahmood Ahmed, who may have been reluctant to join an American-led coalition against the Taliban government that his organization helped bring to power.

Still, American officials said the depth of support within elements of the I.S.I. for a war on the Taliban and Al Qaeda remained uncertain, and a former chief of the agency has become one of the most vocal critics of American policy in Pakistan.

The former director general, Hameed Gul, complained in an interview with a Pakistani newspaper that the Bush administration was demanding that the agency be placed at the disposal of the Americans, as if it were a mercenary force.

"The I.S.I. is a national intelligence agency, whose potential and ouput should not be shared or rented out to other countries," Mr. Gul said.

American officials acknowledged that recent American policies toward Pakistan had fueled such attitudes. In the 1990's the Central Intelligence Agency failed to maintain the close ties it had developed with the I.S.I. in the American agency's covert action program to support the Afghan rebels fighting the Soviet army of occupation in the 1980's.

The close personal relationships that had developed between C.I.A. and I.S.I. officials - General Gul among them - during the war against the Soviets withered away.

"After the Soviets were forced out of Afghanistan," said Shamshad Ahmad, Pakistan's ambassador to the United Nations and a former foreign secretary, "you left us in the lurch with all the problems stemming from the war: an influx of refugees, the drug and gun running, a Kalashnikov culture."

In recent years, in fact, American officials said, the United States offered few incentives to the Pakistanis to end their relationship with the Taliban. Washington gave other issues, including continuing concerns about Pakistan's nuclear weapons program and its human rights record, much greater emphasis than the fight against terrorism.

Those priorities were illustrated by the apathetic reaction within the United States government to a secret memorandum by the State Department's chief of counterterrorism in 1999 that called for a new approach to containing Mr. bin Laden.

Written in the the wake of the bombings of two embassies in East Africa in 1998, the memorandum from Michael A. Sheehan, the State Department's counterterrorism coordinator, urged the Clinton administration to step up efforts to persuade Afghanistan and its neighbors to cut off financing to Mr. bin Laden and end the sanctuary and support being offered to Al Qaeda.

Mr. Sheehan's memo outlined a series of actions the United States could take toward Pakistan, Afghanistan, Saudi Arabia, the United Arab Emirates and Yemen to persuade them to help isolate Al Qaeda.

The document called Pakistan the key, and it suggested that the administration make terrorism the central issue in relations between Washington and Islamabad. The document also urged the administration to find ways to work with the countries to curb terrorist money laundering, and it recommended that the United States go public if any of the governments failed to cooperate.

Mr. Sheehan's plan "landed with a resounding thud," one former official recalled. "He couldn't get anyone interested." As the threat from Al Qaeda and Mr. bin Laden grew and the United States began to press Pakistan harder to break its ties to the Taliban, the Pakistanis feigned cooperation but did little, current and former American officials say.

One former official said the C.I.A. "fell for" what amounted to a stalling tactic aimed at fending off political pressure. The C.I.A. equipped and financed a special commando unit that Pakistan had offered to create to capture Mr. bin Laden. "But this was going nowhere," the former official said. "The I.S.I. never intended to go after bin Laden. We got completely snookered."

The C.I.A. declined to comment on its relationship with the Pakistani agency, saying it did not discuss its ties with foreign intelligence services. But a former senior Clinton administration official disagreed with the idea that the United States had had unrelaistic expectations about the commando proposal.

"There were some concerns about the penetration of the I.S.I., and a lot of uncertainty about whether it would work," the official said. "But all of us, including the intelligence community, thought it was worth doing. What was there to lose?"

What is most remarkable about the tensions that have grown in recent years between the United States and Pakistan's security service is that it was one of the C.I.A.'s closest allies just over a decade ago.

In the 1980's, when the C.I.A. mounted the largest covert action program in its history to support Afghan rebels against the Soviets, the Pakistani agency served as the critical link between the C.I.A. and the rebels at the front lines.

While the C.I.A. supplied money and weapons, it was the I.S.I. that moved them into Afghanistan. The Americans relied almost entirely on the Pakistani service to allocate the weapons to the rebel leaders, and the senior C.I.A. officials involved developed close relations with their counterparts.

But when the Soviet Army finally pulled out of Afghanistan in 1989, the C.I.A. ended its support for the Afghan rebels, the agency's relationship with the Pakistani agency was neglected and Washington began to complain more openly about the Pakistan's nuclear weapons program.

By the early 1990's, officials of the Pakistani agency became resentful over the change in American policy. In 1990, just one year after the Soviets pulled out of Afghanistan, Congress imposed sanctions on Pakistan for its nuclear program.

Faced with turmoil in post-Soviet Afghanistan - which the United States had no interest in addressing in the early 1990's - Pakistan moved in to support the Pashtun ethnic group in southern Afghanistan as it created the Taliban movement.

With Pakistani support, the Taliban gradually took control of most of the country. By 1996, Mr. bin Laden, who had been in Afghanistan in the 1980's, helping to pay for Arab fighters to battle the Soviets, returned and quickly forged a close alliance with the Taliban.

American officials do not believe that the I.S.I. was ever directly involved with Mr. bin Laden and Al Qaeda in terrorist activites against the United States. But the Pakistani agency used Afghan terrorist training camps for its Kashmiri operations, and the Pakistani leadership failed to act as it watched the the relationship between Al Qaeda and the Taliban grow ever closer.

The I.S.I. did cooperate with the C.I.A. and the F.B.I. on several counterterrorism operations in the 1990's. Most notably, the Pakistanis were instrumental in the capture in Islamabad in 1995 of Ramzi Yousef, the mastermind of the first World Trade Center bombing in 1993, and the arrest in Pakistan in 1997 of Mir Aimal Kansi, who killed two C.I.A. employees on a shooting rampage outside C.I.A. headquarters in 1993.

American officials now believe that the Pakistanis were finally starting to become alarmed in the last year or two by the extent to which the Taliban had been co-opted by Mr. bin Laden. Still, the I.S.I. did little to extricate itself from its relationship with the Taliban - until Sept. 11.

"I think the Pakistanis realized as time went on that they had made a bad deal," one State Department official said. "But they couldn't find an easy way out of it."