The Soviet Union has recently conducted several tests of long-range cruise missiles, and U.S. intelligence experts say they believe theywere launched from the Backfire bomber, administration sources said yesterday.

They said the air-launched cruise missiles streaked for 1,200 miles under their own power, about twice the ranged the Soviets had demonstrated in earlier tests.

The probability that the new Backfire bomber was used as the launch platform for the longer-range missiles loomed as a potential complication in a new strategic arms limitation treaty (SALT) between the United States and the Soviet Union.

Under the current arrangement, the plane would not be counted against the ceiling of 2,250 strategic launch vehicles the Soviets will be allowed, since it is not supposed to be deployed in a manner threatening to the United States.

Earlier this week. Defense Secretary Harold Brown told the House Armed Services Committee that if the plane were armed with long-range missiles, it would have to be counted as a strategic weapon, just as the heavier Soviet bombers, the Bison and the Bear, and the U.S. B 52 counted.

Since development of the Backfire was first detected 10 years ago, the Soviets have deployed about 400 of them. About 30 are produced each year. Administration officials who, confirmed the long-range missile firing yesterday said they believed the Backfire was used as the launch platform, although they said they could not be certain.

Some experts on the SALT II negotiations, however, were skeptical. They said it would make no sense for the Soviets to use the Backfire and intersify an already sharp controversy when they could just as well us one of their heavy bombers to test a longrange cruise missile.

Officials also confirmed yesterday that the Soviet Union had encoded engineering data returned from a test flight of a Soviet SS18 ICBM Dec. 21.

The test took place the day that Secretary of State Cyrus R. Vance and Soviet Foreign Minister Andrei A. Gromyko met in Geneva in an effort to conclude SALT II.

According to well-placed sources, Vance and the U.S. delegation objected to the encoding of the test data, saying it impeded U.S. efforts to monitor Soviet missile tests for compliance with arms agreements.

Though the meeting hit a snag, apparently because of the U.S. normalization of relations with China, and did not conclude the treaty, U.S. officials said the encoding issue was settled to the satisfaction of the United States.

In the treaty being completed, U.S. officials said, the Soviets agree they will not encode any missile test data bearing on the United States' right to verify compliance with the agreement.

The verification issue has been one of the most debated problems of the long negotiation, and it promises to be one of the central controversies in Senate debate on approval of SALT II.