Soviet Leader Leonid I. Brezhnev is unlikely to visit the United States for a summit meeting with President Carter this year unless progress is being made on a new nuclear arms treaty, a leading Soviet official said here yesterday.

However, the official said he believes it "possible" that Brezhnev and Carter could meet in a "third" country, probably in Europe, "if developments improve" between the Soviet Union and the United States.

The phrase seemed vague enough to apply to almost any substantive progress inthe stalled arms control negotiations. The present interim strategic arms limitation agreement, signed in Vladivostok in 1974, is scheduled to expire Oct. 1. The Soviets rejected two new SALT proposals made last month by Secretary of State Cyrus R. Vance in Moscow.

President Carter recently said he hoped he could meet Brezhnev for a "get-acquainted" session regardless of unresolved arms control questions.

Vance and Soviet Foreign Ministr Andrei A. Gromyko are scheduled to meet May 18 in Vienna to attempt resumption of the talks.

One rejected U. S. proposal would have reduced the number of strategic bombers and missiles, set at 2,400 by the Vladivostok agreement, to between 1,800 and 2,000. An alternative U. S. proposal - also rejected - would have ratified the 2,400 figure, but it ignored the question of limiting either the cruise missile, a potent new U. S. weapon now in advanced testing stages, or the new Soviet Backfire bomber U.S. military experts say the Backfire is a strategic weapon.

The Soviet official said flatly yesterday: "We would prefer no cruise missile at all." He refused to let his name be used.

The Soviet official summed up the Vance proposals this way: "We scrap some weapons and the Americans can build thousands of cruise missiles. It was seen by us as a package that we had to reject."

He maintained that U.S. press reports had portrayed the Soviets as unable to understand the Vance proposal. He declared: "I assure you, it was rejected because we understood it too well . . . one-side, unilateral advantages for the U.S."

Another Soviet official, visiting here at an informal gathering earlier this week, remarked, "It would have been a sign of weakness if we had accepted any of it."