A senior Soviet official, citing sweeping shifts in the European political landscape, has proposed a drastic reduction in U.S. troops and allied military equipment allowed to be based there under a new conventional arms treaty due to be completed this year, according to U.S. and allied officials.

Chief Soviet negotiator Oleg Grinevsky, noting the forthcoming withdrawal of all Soviet military forces from Eastern Europe, told U.S. treaty negotiators that the United States should be restricted to 70,000 to 80,000 troops in Western Europe, the officials said.

He also has proposed halving the number of tanks, armored personnel carriers and artillery pieces that the West wants to keep in Central Europe under the new accord.

This new proposal would force a U.S. troop reduction of 75 percent from the current level of roughly 305,000.

It is well below the total agreed by both superpowers in Ottawa last summer of 225,000 U.S. troops in Western Europe and 195,000 Soviet troops in Eastern Europe, the Western officials said.

"This may be more of a trial balloon than a serious proposal," a senior U.S. official said yesterday. "It also may be a reflection of the confused state of Soviet arms negotiating."

U.S. officials say that the Bush administration plans to seek clarification of Grinevsky's proposal, which they called unacceptable, and to tell Moscow that it would jeopardize completion of the treaty in time for a planned East-West summit meeting in Paris during November.

The proposed treaty is expected to be discussed briefly during the Helsinki summit meeting today between President Bush and Soviet President Mikhail Gorbachev, and at greater length by aides to Secretary of State James A. Baker III and Soviet Foreign Minister Eduard Shevardnadze in Moscow this week.

U.S. officials say Grinevsky appeared to be backing away from Shevardnadze's pledge to Baker in Irkutsk last month that all U.S. and Soviet troop limits could be dropped from an initial conventional arms accord, a proposal that the Bush administration enthusiastically embraced.

They said Grinevsky had evidently picked the limit of 70,000 to 80,000 U.S. troops because it corresponds roughly to an Army corps, the minimum that Joint Chiefs of Staff Chairman Colin L. Powell and Senate Armed Services Committee Chairman Sam Nunn (D-Ga.) have suggested should remain in Europe over the long term.

Nunn, endorsing the views of former U.S. defense secretary James R. Schlesinger, said in April that "we should start planning on a residual force in Europe on the order of 75,000 to 100,000 within five years."

A U.S. official, while acknowledging some domestic support for the idea of such drastic cuts, said, "We're not prepared to announce a level that low at this particular time." He said it was widely understood that U.S. forces will eventually drop well below 225,000, but that any U.S. commitment to such a low number was impossible before a second conventional forces treaty, to be negotiated later. "Our position is that we do not envision any specific limits on U.S. personnel" in the accord to be completed this year, he said on condition that he not be identified.

In explaining the new Soviet proposals on military equipment in Central Europe -- a zone that includes the territory of six nations besides Germany -- Grinevsky said recently that the planned withdrawal of Soviet forces from Eastern Europe had "radically" changed projections of the region's security structure.

He said the Western numbers, which Moscow had earlier accepted informally, would give the Western forces an advantage of roughly 2 to 1 in the area that once marked the front line of East-West discord.

"What should we do in this situation? Should we disregard it and pretend that nothing is happening and leave everything unchanged?" Grinevsky said in a negotiating session Aug. 30. "But this could be like the three oriental monkeys covering their eyes in one case, ears in the second case, the mouth in the third case as if illustrating the formula, see nothing, hear nothing, say nothing to anyone."

Under the new Soviet proposal, the West would be allowed to keep equipment in Central Europe roughly comparable to that held by former Soviet allies after all Soviet forces have been withdrawn. Grinevsky said this meant a limit of 4,000 to 4,500 tanks, up to 6,000 armored personnel carriers and 3,500 artillery pieces. The West has proposed 8,000 tanks, 11,000 armored personnel carriers and 4,500 artillery weapons for this region.

Grinevsky also proposed for the first time that up to 80 percent of the equipment allowed in the eastern zone covered by the treaty -- which in the current negotiations technically stretches from the German border eastward to the Soviet Ural Mountains -- could be owned and operated by his country. The North Atlantic Treaty Organization had proposed a 60 percent limit, but Moscow evidently feels its former allies do not plan to maintain sufficient equipment of their own to balance the Western forces.