The Venice summit participants reacted cautiously today to the Soviet Union's announcement of a partial troop withdrawal from Afghanistan, saying it could be a "useful" step only if it leads to a total pullout.

In a statement issued on behalf of President Carter and the leaders of six other industrialized democracies, the summit participants said the "Soviet invasion of Afghanistan "undermines the very foundations with efforts to maintain genuine detente."

"In order to make a useful contribution to the solution of the Afghan crisis, this withdrawal, if confirmed, will have to be permanent and continue until the complete withdrawal of the Soviet troops," the leaders said.

The Soviet's public announcement of their intention to withdrawl "some" troops from Afhanistan came in a brief, six-line dispatch carried by the Tass news agency early this morning. Although it had been conveyed privately to the French before the public announcement, the step apparently came as a surprise and served to focus the attention of the leaders gathered here on a new Soviet initiative, however vague its intent.

The major Western nations have been united in their call for a complete pullout of an estimated 80,000 to 100,000 Soviet troops from Afghanistan since the Soviet invasion in December. But the Western allies have been divided on how to achieve that end, with splits developing on a U.S. call for a boycott of the Moscow Olympics and for economic pressure against Moscow.

The President echoed the cautious but hopeful tone of the joint statement in an impromptu news conterence outside the Cipriani Hotel, where he is staying during the Venice economic summit conference that formally began today.

"If these [withdrawal] reports are confirmed, and if these withdrawals are permanent, and if these withdrawals are just a first step in moving the Soviets toward a permanent and total withdrawal of their forces from Afghanistan, then of course this will be a significant step," Carter said.

But the president said he had no way of knowing whether the troop withdrawals represented the beginining of a real pullback by the Soviets.

Carter also said he believed the Soviet announcement had "much more" to do with a desire to encourage participation in the Summer Olympics in Moscow than to interfere with or influence the summit conference of the United States and its principal allies from Western Europe, Canada and Japan.

The initial American reacton to the Soviet announcement was openly skeptical.

"Don't believe anything you don't see," Secretary of State Edmund Muskie said when asked about the report.

"This is not the same thing as pulling out Soviet troops from Afghanistan," said White House National Security Adviser Zbigniew Brzezinski. "We don't have precise data, [but] it could be routine troop rotation."

Tonight, a senior American official who asked not to be indentified said the withdrawal appeared to involve "several thousand" troops.

"I don't want to dismiss this withdrawal as irrelevant, but it must be confirmed and continuous," he said.

[A Tass statement late Sunday called the U.S. comments on the Soviet announcement "slanderous" in tone.]

The joint statement on behalf of the seven leaders was read to reporters by Italian Prime Minister Francesco Cossiga, host of the two-day summit conference that will conclude Monday.

Despite the strong language of the statement condemning the Soviet invasion as "unacceptable" and as a threat to peace, it made no reference in it to the kind of "concrete" display of Western opposition to the Afghan occupation that Carter has called for.

Cossiga said the government at the summit that had supported Carter's call for a boycott of the Moscow Olympics "vigorously reaffirm their positions." All of the goverments except France, which was officially neutral on the question, urged a boycott, but that was not enough to maintain unity among the summit nations. In addition to France, Britain and Italy will send teams to the Olympics in accordance with the votes of their independent national Olympic committees.

Instead of specifics, the statement said the summit nations "are resolved to do everything in our power to achieve [the] objective" of a complete withdrawal and to "support any initiative to this end."

Neverless, today's joint declaration clearly served Carter's purposes. The president came to Venice stressing the need for unity among the allies in response to the Afghanistan invasion, and that was the theme expounded by him and his senior aides after the statement was issued.

"This was a carefully worded statement that shows the unity among the seven nations assembled here. . . ." the president told reporters.

White House Press Secretary Jody Powell later added that the statement was "a sign and a signal of a unified position on this very important issue."

In other statements, the Venice summit nations:

Condemned the taking of diplomatic hostages but did not mention the American hostages in Iran. Powell dismissed questions on this omission, saying the United States had not asked for a specific reference to the Iran stalemate and asserting that the statement's meaning was "fairly obvious."

Declared that the worldwide problem of refugees must be "attacked at its root" and said that the governments responsible for the refugee problem should "remove the causes of this widespread human tragedy and not pursue policies which drive large numbers of their people from their own homes." The statement mentioned specifically refugees from Indochina, Afghanistan, Cuba and Africa.

Political rather than economic topics dominated the first day of the summit as the leaders gathered on San Giorgio Island directly across the waters from Venice's famed San Marco Square.

Carter began the day by attending a Catholic mass celebrated in Italian at the Church of St. Eufemia, located about a mile down the Grand Canal from the summit site.

He than attended a breakfast with the other leaders before the summit officially opened with an initial session on economics and energy. After lunch together, the leaders held a more than an hour meeting on political subjects that was donimated by the Afghanistan issue.

Tonight, before all the leaders gathered for dinner, the president met privately for more than an hour with French President Valery Giscard d'Estaing. Little information was availabe on the meeting, which Carter said "could not have gone any better."

In other comments on the Soviet announcement a spokesman for British Foreign Secretary Lord Carrington said that "only the complete withdrawal of all Soviet forces can bring peace and stability to the area."

He added, "The timing of the terse Soviet announcement is transparent. The facts are oapque."

A spokesman for West German Chancellor Helmut Schmidt said, "Maybe it's just a token. . . It would be good if it's serious."