The new East-West treaty on conventional forces in Europe will require destruction of far fewer Soviet weapons than once anticipated because Moscow has withdrawn many of its forces from the region covered by accord before it takes effect, U.S. and diplomatic officials said yesterday.

A U.S. military official said the Soviets had redeployed "tens of thousands" of arms east of their Ural Mountains in what appears to be a deliberate effort to exempt them from the treaty provisions. The unregulated movement of tanks, artillery, planes and armored combat vehicles is permitted while the negotiations continue but must cease after the new treaty takes effect.

Secretary of State James A. Baker III said yesterday that nations belonging to the former Warsaw Pact military alliance will have to destroy 19,000 tanks under the accord. That number is roughly 13,000 fewer Soviet tanks than NATO officials predicted last year eventually would have to be destroyed. Officials said the difference is caused by the smaller number of tanks now in the zone between the former inter-German border and the Soviet Urals.

Asked about the Soviet arms movements, Baker said, "we have asked for and been receiving some accountings with respect to what they are doing." He said the Soviet effort "points up . . . the significance of getting an agreement in principle" Wednesday on the treaty provisions, ensuring that the treaty will be signed as planned in two months and abbreviating the period in which the Soviets can legally move their forces in the area.

Other officials said Soviet Foreign Minister Eduard Shevardnadze and his aides had provided assurances that Moscow is not attempting to develop a new "strategic reserve" of conventional, or non-nuclear, arms east of the Urals, and pledged that some of the weapons being deployed there would be unilaterally destroyed.

President Bush said yesterday that the treaty "would decisively improve the balance of military power on the continent and back our hopes for lasting stability."

Baker and others noted that the West had informed the Soviets this week of its weapons redeployments in Europe, including shipments of modern U.S. arms to allies whose weapons will be destroyed under the accord. "We are trying to ensure that the oldest weapons get replaced with the newest" to gain maximum efficiency from the arsenal allowed to remain, a senior U.S. official said. "This means, for example, that we will probably get rid of all the Portuguese tanks and give them some new ones."

Baker said at a White House news conference that the treaty would limit Eastern and Western forces each to 20,000 tanks, 20,000 artillery pieces, 30,000 armored combat vehicles and 2,000 helicopters. He declined to specify a number for combat aircraft, explaining that it "is still subject to some discussion with allies on both sides."

Baker also said the treaty would allow roughly two-thirds of the tanks, artillery and combat vehicles on each side to be owned by the United States and the Soviet Union, while 75 percent of the combat helicopters could be owned by one of the superpowers. Under the treaty, the Western forces are those held by the 16 nations belonging to NATO, while the Eastern forces are those of the six nations that once belonged to the Warsaw Pact plus the forces remaining on former East German territory.