ISLAMABAD, PAKISTAN -- Differing assessments of the strength of the Kabul government without the support of Soviet troops may well lie behind the diverging U.S. and Pakistani approaches to an Afghan peace accord.

New U.S. views of the strength and support for the government of President Najibullah in Kabul indicate that it could crumble quickly without the active backing of the 120,000 Soviet troops now in Afghanistan, according to informed sources.

Pakistani officials, however, stressed last week that they want to consult with Washington to reach what one source called "a more reliable assessment of prospects" for Afghanistan after a Soviet withdrawal. A diplomat interviewed here suggested that Pakistan worries that the accord being crafted under U.N. auspices would allow the Soviet Union to continue supplying Afghan government forces, while U.S. and Pakistani aid to the mujaheddin guerrillas would be cut off. Pakistan worries that an imbalance in supplies could help the Afghan government forces prolong their fight against the guerrillas, an argument that assumes that there will be an Afghan Army to fight for some time.

Officials in Pakistan, which shelters mujaheddin and an estimated 3 million Afghan refugees, have expressed fears that continued warfare in Afghanistan would prevent the exiles from going home, and could destabilize Pakistan. Since last month, Pakistan has been arguing that an interim government must be established as part of the agreement, which now calls simply for a Soviet pullout and a cutoff of aid to the guerrillas.

Last week, Soviet Foreign Ministry spokesman Gennadi Gerasimov accused Pakistan of trying to sabotage the agreement with its new insistence on an interim government. U.S. officials said that Secretary of State George P. Shultz will raise the issue in his talks in Moscow this week -- but there appears to be less enthusiasm for pressing the Kremlin on an interim government than on the issue of whether Moscow will be permitted to continue supplying the Afghan government forces.

A key Pakistani official said, "Our position is based on the objective realization that fighting must cease or the accord cannot be implemented. Kabul doesn't control half {of Afghanistan's} territory, so how can they implement it? Even Soviet diplomats say in private conversations that the {guerrilla} alliance controls at least 50 percent of the territory. There is a certain parity."

Pakistani officials do not speak openly about how long they think the Afghan government might be able to continue the war, but one well-informed diplomat appeared to mirror Islamabad's fears.

"The mujaheddin and the United States are victims of their own propaganda that Najibullah is a puppet who will collapse without the Russians," the diplomat said. "On the basis of the present {U.N.} accords, the Najibullah government would receive substantial Soviet aid . . . . so the war would continue with aid cut off from this side. What do the refugees do?"

Estimates here of the strength of the Afghan military vary widely, but there seems to be some consensus that there has been an effort to rebuild it in recent months. Different sources estimated that the regular Army numbers between 50,000 and 100,000 troops. They estimated that there are similar numbers of paramilitary forces and several thousand members of the secret police.

The Soviets could be expected to leave behind materiel, and, under the U.N. formula, still send new supplies. An Afghan diplomat said recently that Kabul intends to keep this avenue open. When Soviet Deputy Foreign Minister Yuli Vorontsov was asked about the resupply issue in Pakistan recently, he is reported to have maintained silence.

The United States appears to believe that the Afghan Army is likely to melt away as the Soviet troops leave. "The streets of Kabul will be littered with uniforms as these guys go back to their villages," said one diplomat. Other diplomats suggested that only the secret police would likely be loyal to Najibullah. They argued that a Soviet pullout would soon leave him in control of only the capital and its region, and that this would quickly collapse.

Other analysts argue that Najibullah's forces will prove more cohesive and willing to fight and will be able to confront mujaheddin cut off from supplies and possibly divided.