While President Clinton was in Geneva earlier this month making a speech to the International Labour Organization, his national security adviser, Samuel R. "Sandy" Berger, slipped out to receive an alarming letter from Indian Prime Minister Atal Bihari Vajpayee.

Vajpayee's message was that India might have to attack inside Pakistan if Pakistan did not pull back troops who had seized Indian outposts in the disputed territory of Kashmir.

It stoked already high U.S. fears that India, which has lost more than 100 troops trying to dislodge the Pakistanis, would storm across the cease-fire line that divides Kashmir or open a second front elsewhere on its border with Pakistan, widening the first armed conflict between the rivals since both tested nuclear weapons last year.

Clinton faced the possibility that some 700 Pakistani troops, camped on firebases 16,000 feet up in the Himalayas, could destroy the rapprochement India and Pakistan began last winter, ignite a regional war and scuttle the administration's dwindling hopes of a constructive new relationship with all of South Asia.

The president and Secretary of State Madeleine K. Albright had already sent multiple messages urging restraint to India and Pakistan. After Berger and Karl F. Inderfurth, assistant secretary of state for South Asian affairs, received Vajpayee's message in Geneva, Clinton decided to turn up the pressure on Pakistan, senior officials said.

First he persuaded the so-called G-8 countries to include in their final communique last weekend a statement condemning the "infiltration of armed intruders" and demanding "full respect" for the de facto border known as the Line of Control. The statement did not call for a cease-fire, an implicit acknowledgment of India's right to defend its territory.

Then Clinton dispatched to Islamabad Gen. Anthony Zinni, commander of the U.S. Central Command, to tell Prime Minister Nawaz Sharif and his military commanders to pull back to their side of the Line of Control. What happens next will depend on Zinni's assessment of Pakistan's response, one senior official said, noting that impoverished Pakistan cannot afford full-scale war and is counting on receiving a $100 million loan next month from the International Monetary Fund. Washington could hold up those funds to pressure Pakistan.

"We're not making any predictions," a senior administration official said. "It could get worse, if the Indians reach the level of frustration that they need to strike somewhere else." That concern rose after Vajpayee's letter, officials said, because it told Clinton that the spectacle of Indian troops coming down from the mountains in body bags was raising public pressure on the government.

Pakistan's incursion into Kashmir has turned U.S. diplomacy upside down. A year ago, the Clinton administration was orchestrating an international campaign of condemnation against India after Vajpayee's government tested nuclear weapons. Pakistan followed with its own tests, but Washington expressed understanding of the pressures that led to that decision and looked for ways to ease the mandatory U.S. economic sanctions triggered by Pakistan's tests.

Now India is drawing praise for its restraint in the Kashmir conflict, and it is Pakistan that is being criticized. After initially promising public neutrality, Clinton has authorized U.S. officials to say there is no doubt that the intruders on the Indian side of the line are Pakistani regulars, as India claimed, not Kashmiri separatist guerrillas, as Pakistan said.

There may be a handful of the Islamic militant irregulars known as Mujahedin with the troops, one senior official said, but most of the invaders are regulars from the 10th Corps of the Pakistani army.

"Pakistan is the instigator here," a senior administration official said. "Pakistan has to figure out how to restore the status quo ante." U.S. officials said they do not expect the 50-year dispute between India and Pakistan over which should possess Kashmir to be resolved soon. But they insist that Pakistan pull its troops back across the cease-fire line and return to the negotiating table.

Quick compliance is essential, U.S. officials said, because the road cut off by Pakistani shelling, the only supply route to two Indian towns, will be closed by snow after early September, and India is determined to clear it before then.

Last year's nuclear tests and the Kashmir fighting have shredded administration hopes of overcoming the Cold War legacy of distrust in South Asia and forging cooperative political and economic relationships with India and Pakistan. Hopes rose in February when Vajpayee celebrated the opening of cross-border bus service by riding to Lahore, Pakistan, for a meeting with Sharif at which they pledged to settle their differences amicably. But the Kashmir fighting has brought the two closer to war than to peace.

"This has been enormously disappointing," Inderfurth said. "We didn't think the next stop on the diplomacy bus would be Kargil," the mountain town at the center of the fighting.

U.S. intelligence analysts have offered several reasons why Pakistan would risk international condemnation, and war with a nuclear-armed neighbor, for a relatively minor territorial advantage.

One is an attempt to force the Kashmir issue into an international forum such as the United Nations. Another possibility is that senior Pakistani military officers wanted to abort the Lahore peace process, although some U.S. officials discount that theory, saying Sharif cleared the Lahore initiative with the armed forces leadership.

U.S. officials also share India's concern that Pakistan's top military officers are attempting to export into Kashmir the same type of rigid Islamic orthodoxy imposed on Afghanistan by the Pakistan-backed Taliban militia.

Mohammad Aziz, chief of the general staff, and Gen. Pervez Musharraf, army staff chief, "have spent their careers supporting one mujaheddin movement after another," one senior official said. Their appointments in a recent military shake-up "raise serious questions about the long-term direction of the Pakistani state," the official said.

"We don't want the Talibanization of Kashmir," said Indian Ambassador Naresh Chandra, "but if you use these guys as guest terrorists of the Pakistani army, what would be the consequences?"