Western European allies, while expressing reservations about specifics, have voiced overall approval of President Reagan's message on arms control to Soviet leader Mikhail Gorbachev, officials and diplomats in several European capitals said this week.

Based on a draft of the Reagan response discussed with them this week by arms control adviser Paul Nitze, officials here and in London said that the U.S. administration appeared willing to commit itself to continued adherence to the 1972 Antiballistic Missile (ABM) Treaty for a period of at least several more years.

The reported U.S. position would narrow the gap between the two superpowers' negotiating stances. Moscow proposed last month to extend the ABM Treaty by at least 15 years, in exchange for 30 percent cuts in both superpowers' strategic nuclear arsenals.

The allies have interpreted the reported U.S. proposal to mean that Washington is willing to ease its refusal to negotiate over any aspect of the "Star Wars" missile defense system, which is formally known as the Strategic Defense Initiative (SDI). The West Europeans have long maintained that testing, development and deployment of SDI should be subject to consultation with them and to negotiation with the Soviets.

Reagan sent a letter to Gorbachev Friday, administration officials said in Washington, proposing not to deploy "Star Wars" for seven years in return for deep reductions in offensive weapons by both sides and for Moscow's eventually acceding to some form of space-based missile defense.

The allies were not in total agreement with the U.S. draft letter, the officials and diplomats said. They have quietly urged Washington this week to sweeten the message to Gorbachev by making stronger commitments to respect the ABM Treaty and the unratified 1979 SALT II nuclear arms pact, they said.

The allies also have discreetly urged Washington to accelerate attempts to reach a comprehensive nuclear test-ban treaty. The Soviet Union is seeking talks on that issue, and has observed a unilateral moratorium on nuclear tests for nearly a year.

U.S. and Soviet arms experts began talks in Geneva today on what a joint U.S.-Soviet communique called "the entire scope of issues related to nuclear testing." The two sides entered the meeting with differing approaches, however.

The United States planned to seek improvements in verification measures for two nuclear test ban treaties that were signed in the 1970s but never were ratified. The Soviets planned, instead, to repeat their call to begin talks now on an overall test-ban treaty.

The British government favors efforts "in every way possible" to achieve a comprehensive test-ban treaty, but considers cuts in superpower offensive arsenals to be the priority. In the meantime, it feels that testing must continue to ensure that existing weapons and new systems are effective.

West German officials welcomed the opening of the Geneva test-ban talks and said that Bonn favored a "speedup" of efforts to reach a comprehensive pact. The government here has been uncomfortable with U.S. resistance to entering such negotiations, although it has recognized the need for adequate verification measures and has sought to avoid an open disagreement with Washington on the issue.

West Germany also asked the United States to devote more attention to the need for reductions in conventional arms in Reagan's letter to Gorbachev.

But allied officials emphasized their satisfaction with the overall thrust of Reagan's message, as outlined to them by Nitze. The sources indicated that the U.S. proposals, as they understood them, seemed to offer enough to Gorbachev to get the two superpowers moving toward an arms control pact even though substantial differences remained between the two sides.

"We have the feeling that, for the very first time, the two world powers have similar ideas on how the problem can be solved. This does not mean that they are very close in the details, but they have a similar way of thinking. From our point of view, this is encouraging," a West European official said.

A British official said, "Our attitude is broadly welcoming."

An important, unresolved disagreement between the two superpowers concerned the degree to which the ABM Treaty prohibits development and testing of antimissile defense systems such as SDI, allied officials said.

The Soviets have argued that the treaty bars any testing or development of such a system outside of laboratory research. The United States has said that all aspects of SDI, short of actual deployment, are allowed under the treaty and are not subjects for separate negotiation.

"The Soviets will not be able to accept nuclear weapons reductions if they have no assurance that, at least for some period of time, there will be no deployment of strategic defenses," a West German official said. He said that the Bonn government was urging a "workable compromise" on this issue, which he called central to prospects for an arms control pact and a Reagan-Gorbachev summit later this year.

Nitze's trip also included stops in Paris, Rome and Brussels. The West European reaction this week was much more positive than the response to a similar trip in February when Nitze was carrying U.S. proposals regarding reductions in medium-range nuclear missile arsenals, officials and diplomats said.

"In this case there has been general support and no outcries," said a western diplomat who was familiar with the latest proposal carried by Nitze.

The Western European allies have differed with the United States on a variety of arms control issues. They have generally supported a stricter interpretation than Washington of what is allowed by the ABM Treaty, and they criticized Washington's announcement in May that it no longer felt obligated to stay within the SALT II limits on strategic nuclear arsenals.

Diplomats and other observers cautioned that the allies might be playing down their differences with the reported approach in the Reagan letter in part because of a desire to avoid the impression that they were endorsing Moscow's positions.

In addition, the allies apparently wished to be supportive of U.S. positions to avoid a conflict with Washington that might provoke a backlash by U.S. administration hard-liners, according to the diplomats and other observers.Washington Post correspondent Karen DeYoung contributed to this article from London.