BRUSSELS, FEB. 23 -- U.S. intelligence reports that the Soviet Union is already starting to move toward the exits in Afghanistan tend to confirm the beliefs of Secretary of State George P. Shultz that Soviet forces will really withdraw from that eight-year-old war, probably by the end of 1988, senior State Department officials said today.

Members of the Shultz party, which stopped here en route to Washington to brief the North Atlantic Treaty Organization on two days of intense diplomacy in Moscow, said the intelligence indicates that Soviet troops are being garrisoned "in a more defensive way," and some facilities are being dismantled, apparently in preparation for the trip back home.

Other reports suggest that Soviet dependents are being withdrawn and that Soviet advisory personnel who leave Afghanistan are not being replaced. One senior U.S. official cited such reports as concrete evidence of the Soviet intention to withdraw irreversibly.

Shultz, in a Moscow press confernce Monday night after the conclusion of his talks with Soviet leader Mikhail Gorbachev and Foreign Minister Eduard Shevardnadze, said, "I don't have the slightest doubt that the Soviet Union decided it wants to leave Afghanistan."

He added, "We think it will be a good thing if Soviet troops are out of Afghanistan by the end of this year and the refugees are able to return to their country and the citizens of Afghanistan can construct a government to their liking . . . . I feel there is a very good chance of that happening."

Aides said it was the strong statements by Gorbachev and Shevardnadze, rather than the intelligence reports, that brought Shultz to these conclusions.

Shultz and his aides did not obtain in Moscow the details they had sought about the timing, phasing and ground rules of a Soviet withdrawal.

While this was initially a disappointment, senior officials said today they believe the Soviets withheld the data because they fear news leaks that could lead to a collapse of the pro-Soviet government in Kabul.

Soon after the Soviet withdrawal terms become firm, according to these sources, pro-Soviet Afghan officials are likely to begin fleeing to safe haven in the U.S.S.R.

Moscow's plans are likely to be revealed in Geneva, where U.N.-sponsored indirect talks are scheduled to resume March 2 between the Kabul regime and Pakistan.

Undersecretary of State Michael H. Armacost, who left the Shultz party today to fly to Pakistan, is likely to discourage that government from maintaining any roadblocks in the way of a quick start on a Soviet withdrawal, officials said.

Pakistan has said it will refuse to sign U.N.-negotiated pullout accords with the Soviet-backed Kabul regime, insisting instead on creation of an interim authority or government acceptable to the Afghan refugees and resistance leaders.

Such a demand could complicate and possibly delay a Soviet withdrawal, which Gorbachev has promised will begin by May 15 if the U.N. accords are signed by March 15.

Having urged the Soviets for years to withdraw promptly from Afghanistan, it would be wrong for the United States to ask them to "stick around" until an agreed political settlement can be forged in that country, a senior official told reporters. "We wouldn't want to miss the bus" of a Soviet withdrawal plan, the official said.

Gorbachev, in his meeting with Shultz Monday, gave additional assurances to Washington about the Soviet withdrawal, according to an account released later by the Soviet news agency Tass.

Tass quoted Gorbachev as telling Shultz that "the Soviet Union never had and does not have any secret plans whatsoever or intentions in respect of Afghanistan like creating some sort of a bridgehead there." The Soviet leader, according to this account, reiterated his Feb. 8 pledge to withdraw all of Moscow's forces following the signing of the Geneva documents.