Defense Secretary Caspar W. Weinberger said here today that President Reagan's "Star Wars" umbrella would protect Europe as well as the United States from Soviet missiles and would not lead to any American "decoupling" from NATO defenses.

By repeating at a U.S. Embassy press conference here his assertions that President Reagan's Strategic Defensive Initiative is not a go-it-alone strategy, Weinberger appeared to be trying to allay fears in Europe about the concept of the space-based missile defenses.

It was not immediately clear if Weinberger's efforts would succeed. European leaders have expressed broad concern that attempts to replace the present system of deterrence built on offensive weapons with a defensive shield based in outer space would destabilize the balance of power between the Soviet Union and the United States.

Weinberger's remarks were devoted to the narrower issue of U.S. willingness to provide Europe with its own shield or a share of the American shield once the system is in place.

"Contrary to what a lot of people have expressed," Weinberger said, "if the initiative works, it will work against intermediate-range missiles as well as intercontinental-range missiles , so there should be no suggestion of decoupling the United States from Europe or anything of that kind."

The defense secretary added that the Reagan administration hopes a number of allied nations will join the United States in working on a missile defense, declaring, "We need all the help we can get." Weinberger rejected the idea of inviting the Soviet Union to join the effort. Reagan has suggested sharing a missile defense with the Soviets if one is perfected.

Weinberger said "it would be a little easier" to defend against intermediate-range missiles "than any others," but he gave no hint of what possibilities the Pentagon is exploring. The shorter range missiles fly lower and take less time to reach their targets than intercontinental rockets.

Richard D. DeLauer, Weinberger's former research chief at the Pentagon, was intrigued with the possibility of using "electric" guns to combat intermediate missiles. The projectiles would be hurled at their targets by the force of an electric charge.

If the Strategic Defense Initiative advances from paper studies and ground experiments to flight tests, the question of whether the United States would be violating the 1972 antiballistic missile treaty could lead to difficulties with the Soviet Union. Weinberger steered around the issue at his press conference today.

"I don't think any amendment is needed to follow on with what we're doing now," he said of the ABM treaty, which prohibits development, testing or deployment of space-based ABM components. Pressed on what his attitude would be toward the treaty if flight testing became necessary to prove the feasibility of missile defense, Weinberger responded that this was getting into the "hypothetical field."

Weinberger reacted coolly to the idea of pursuing an antisatellite weapons (ASAT) treaty with the Soviets to head off the accelerating militarization of space. "I am not aware of any compelling need for an ASAT" treaty or "anything of that kind," he said. "The Soviets have an ASAT system deployed. It is essential, therefore, that the West pursue the capability of deterring the use of that. . . ."

Weinberger left Washington yesterday for a five-day trip to Britain and West Germany. After his press conference today he went to the Ditchley Foundation outside Oxford to participate in round-table discussions on defense issues. Accompanying him from Washington to Ditchley were Rep. Les Aspin (D-Wis.), newly elected chairman of the House Armed Services Committee, which will recommend money ceilings for Pentagon programs to the House, and Sen. John Warner (R-Va.), second-ranking Repulican on the Senate Armed Services Committee.

Aspin and Warner said in separate interviews that Weinberger will have a much harder time this year than last to stave off deep cuts because the Senate is far less supportive of the Pentagon budget. Warner predicted that Congress will end up approving a real growth of 3 percent in defense spending compared with the 13 percent sought by Reagan.

Weinberger will fly to Munich on Sunday to deliver a speech at a conference on national security. From there, he is scheduled to visit U.S. Army troops in West Germany and then return to London for a speech to the Royal College of Defense Studies on Tuesday. Washington Post correspondent Michael Getler contributed to this report.