A senior State Department official warned yesterday that the Soviet Union has started a campaign to use the "Star Wars" issue in the forthcoming Geneva arms negotiations to drive a wedge between the United States and its Western European allies.

Assistant Secretary of State Richard R. Burt forecast, however, that an "alliance consensus" on the space defense plan, called the Strategic Defense Initiative, will thwart the Soviet efforts and actually enhance the U.S. bargaining position in the negotiations scheduled to begin March 12.

Burt and his Pentagon counterpart, Assistant Secretary of Defense Richard N. Perle, appeared before the Senate Foreign Relations Committee to discuss Soviet-American relations and arms control.

The joint hearing of "the two Richards," who in the past were often reported to be in conflict on U.S. negotiating positions, featured friendly words for one another and attempts by both officials to minimize their differences.

Perle said it is "only right and natural" that officials from various agencies and viewpoints differ on recommendations before final decisions are made by President Reagan.

Reports of the bureaucratic cross-purposes are "greatly exaggerated," he said.

Burt said it would be hard to improve on Perle's "eloquent statement." He added, "This administration has its act together on arms control."

Burt seemed more positive than Perle about arms control and the negotiations that are to resume in two weeks.

The State Department official cautioned, however, that "we are now running the risk of another round of euphoria" as the new negotiations near.

The Soviets are in "the early stages of a new 'peace offensive,' " said Burt, adding that "we must guard against the buildup of expectations which the Soviets are surely seeking to manipulate."

Among the signs of a Soviet "peace offensive," according to State Department officials, are Moscow's recent decision to permit international inspection of some of its civilian nuclear facilities and the planned visit here next week by an unusually large and prestigious Soviet delegation headed by Vladimir V. Shcherbitsky, a member of the Communist Party Politburo.

The House of Representatives will be host to the delegation headed by Shcherbitsky, in return for a 1983 U.S. parliamentary visit to Moscow's Supreme Soviet.

Burt testified that the administration has held formal and informal meetings with the NATO allies to "demystify" the Strategic Defense Initiative in order to shore up support for it.

On the same topic, Perle said the Pentagon is studying ways to involve the allies in the research programs leading to a decision on the feasibility of the ambitious program.

Some of the European allies have expressed concern that the SDI plan will work to their disadvantage. The Soviets are expected to emphasize a willingness to make major reductions in long-range and intermediate-range nuclear arms if the United States will curb the SDI and the "militarization of outer space."

Under questioning from Sen. Jesse Helms (R-N.C.), Burt said he was "not aware of" any U.S. agreement to relieve the Soviet Union of its obligation to reduce its strategic nuclear arsenal as called for in the unratified SALT II treaty.

Burt said he understood that Soviet Foreign Minister Andrei A. Gromyko made it "very clear" to then-Secretary of State Alexander M. Haig Jr. in June 1982 that while Moscow would "not undercut" the treaty, it would not make the major reduction in offensive arms called for in the document.

According to the treaty, total strategic offensive arms on each side were to be reduced to 2,250 launchers and bombers in 1981.

Burt, chief of the State Department's bureau of politico-military affairs at the time, described this as a "unilateral" Soviet statement and said he is "not aware of any U.S. agreement." Some conservatives on Capitol Hill have charged that the administration agreed to permit the Soviets to forgo the projected reductions, thus making a "secret executive agreement" with Moscow.

Both Burt and Perle said no decision has been made within the administration on whether to continue the U.S. policy of "not undercutting" the unratified treaty, especially whether to retire some nuclear missiles when a new nuclear-armed Trident missile submarine is deployed in the coming months.

Perle said the Pentagon has taken "necessary actions" to permit continued compliance with the treaty if that is Reagan's decision.

Perle hinted strongly that he opposes such a move, saying that "the time has come" to reject what he charged is a "double standard" of U.S. compliance and Soviet noncompliance with SALT II.

Burt said the decision on continued U.S. compliance will be based on such factors as the record of Soviet compliance, the progress of the forthcoming Geneva negotiations and U.S. military requirements.