Secretary of State George P. Shultz said yesterday the United States must be persuaded that there is a "certain inevitability" and "no turning back" on a Soviet withdrawal of forces from Afghanistan before Washington will cut off military assistance to the Afghan resistance.

He also said any Soviet withdrawal from Afghanistan will have to be "front-end loaded," meaning a larger number of troops leaving at the start, before the United States will honor a commitment it has made in principle to end support for the Afghan rebels once the Soviets leave.

"As far as support in the form of {U.S.} military equipment is concerned, as withdrawal proceeds and as it takes place -- we hope in a peaceful atmosphere -- then you don't have the need for that continued support and it would cease," Shultz said.

This seemed to suggest the United States envisages a process whereby it ends aid progressively, rather than all at once, to assure the Soviet withdrawal proceeds.

{Moscow is stepping up pressure on the Soviet-backed regime in Afghanistan for an early settlement of the civil war.

{On Wednesday, Soviet Foreign Minister Eduard Shevardnadze, in Kabul, the Afghan capital, said his government "would like" 1988 to be the last year of Soviet troops there. Details, Page A16.}

Shultz's comments at a news conference here appeared to be an attempt to more precisely detail the U.S. conditions for an end to American military aid to the rebels, which totaled about $660 million last year.

Shultz outlined U.S. demands and expectations of Soviet behavior during withdrawal of its 120,000 troops from Afghanistan and sought to reassure the Soviets that the administration will end its military aid, as promised, if convinced that the Soviets are actually pulling out.

Shultz also said the United States wants to see an officially "neutral" Afghanistan and linked an end of U.S. military aid to the Afghan resistance with an end of " Soviet assistance to the Afghan government army.

Soviet officials have said they are uncertain whether the Reagan administration is posing new conditions now for a cutoff of U.S. aid and want to know when aid will cease.

Afghan peace accords now being negotiated in Geneva under U.N. auspices stipulate that 60 days after they are signed, Moscow will begin its withdrawal and aid for the rebels will end. The United States has agreed to act as a guarantor of the accords, which await a timetable for Soviet withdrawal.

The United States and Soviet Union, however, have yet to reach an understanding on the point at which U.S. aid for the rebels will end. One U.S. official said the United States had "danced around exactly what we do" until the Soviet Union sets a firm date for withdrawal.

Shultz, responding to Shevardnadze's statement on the Soviet desire not to spend another year in Afghanistan, said, "I hope that turns out to be the case, and, from our standpoint, that would be very desirable.

". . . We look for Soviet agreement to a firm schedule for withdrawal. We think that schedule must be front-end loaded, so that once it starts there's a certain inevitability to it, there's no turning back. Under those circumstances, we will certainly meet the things that we have agreed to in the Geneva process."

Shultz said that Afghanistan will "not in any way {be} aligned with us as a military matter -- not part of a bloc -- in other words, neutral."

"The objective of our support for the resistance has been to bring about those conditions, and, as those conditions emerge, obviously we wouldn't have to continue that {military} support and it would cease," he said.

But he suggested that the United States might still continue humanitarian and economic assistance to the Afghan resistance.

Asked whether the Soviets were obliged to cut off military aid to the Afghan army as part of an agreement, Shultz said, "We would presume that as part of that agreement military supplies would stop going in there." One U.S. official said this is another point Washington and Moscow have yet to resolve.