Soviet Premier Alexei Kosygin has given what visiting U.S. senators see as strong new assurances that the Kremlin considers President Leonid Brezhnev's pledge restricting its supersonic Backfire bombers as part of the SALT II accord to have the binding force of a treaty commitment.

Kosygin, according to those present, told the six Americans today during a Kremlin meeting that the Soviets consider Brezhnev's pledged limitation to be an integral part of the new strategic arms limitation treaty, and if any of the restrictions on combat roles or construction rate are violated, it must be considered a direct treaty violation.

The premier reportedly showed surprise that the U.S. delegation, headed by Sen. Joseph Biden (D-Del.), would question the Backfire assurances Brezhnev made to President Carter in June at the Vienna summit, where the arms treaty was signed.

The assurances consisted of an unsigned written statement restricting the jet bomber's range below intercontinental striking distances and an oral pledge to Carter that not more than 30 planes a year will be produced.

Biden said Kosygin retorted, "Why ask me this question? It's part of the treaty. The Soviet Union has made a treaty, the world knows it, the Western press knows it. You can ask the people in the street. If we build more than 30 Backfires, we violate the treaty."

The Brezhnev Backfire pledges are among the major treaty issues that have sparked opposition within the Senate, which will soon be debating the accord. A ratification vote is expected early this winter.

Kosygin's views as reported by the senators are seen here as a Soviet leadership attempt to find a reassuring public stance on the treaty to improve its chances of passage. Kosygin and senior Soviet officials have said virtually nothing about possible Senate treaty modifications or "clarifications" that could limit sharply the president's observance of some of its provisions.

Biden has interpreted this silence as tacit Soviet acceptance of Senate modifications of the complex treaty package, short of outright amendment. In June at the summit, Brezhnev warned of grave world consequences if the treaty was not accepted as signed by the two presidents. Later, Soviet Foreign Minister Andrei Gromyko issued a similar warning.

While Biden said he was satisfied that Kosygin's statements on the Brezhnev Backfire pledges place the Kremlin unmistakably behind its leader, the other senators were substantially more cautious in assessing the meaning of Kosygin's assertions.

"After the conversation with Kosygin," said Biden, a treaty supporter, "I have no concern about Backfire." He and the other lawmakers held a press conference here prior to departing for Washington via London.

But Sen. Carl Levin (D-Mich.), expressed dismay that the Soviets had not produced a specific written assurance acknowledging Brezhnev's oral pledge on Backfire production limits. The senators did not seek specifically such a written backup from Kosygin, but instead they sought it from senior officials ranking below Brezhnev with whom they conducted most of their three days of talks.

"It was not forthcoming," Levin asserted. He said the Soviet refusal makes "not only likely, but needed," a Senate clarification that any Backfire pledge violation is grounds for U.S. abrogation of the entire SALT II.

Several in the group said the Soviets indicated they could live with such clarifications but would reject any Senate amendments requiring renegotiation of the treaty. Levin said the Soviets ignored his argument that a written Backfire pledge confirmation could help ratification chances in the Senate and would not harm Kremlin interests -- since the commitments already had been made. The necessary two-thirds majority has not emerged in the Senate and Carter faces a stiff fight for approval.

Sen. David Boren (D-Okla.), while asserting he remains undecided on ratification, said he has "no doubt" after the talks here and briefings at NATO headquarters in Brussels that the Soviets seek superiority over the U.S. in conventional arms, tactical theater weapons and counter force strength, and that America "must take action to insure equality" in these areas with the Soviets.

Sen. Richard Lugar (R-Ind.) said Kosygin specifically rebuked him when the senator pressed his view that greater verification of the treaty is needed, which could require amendments and renewed treaty bargaining. Kosygin disagreed, Lugar said, adding that "those who pursue these things would bear a heavy responsibility in a global sense." Lugar said he is "less optimistic" than Biden that the Soviets will accept without objection any Senate clarifications or modifications short of outright amendment, which they are likely to reject.

The senators said about half the talks were devoted to the next strategic arms limitation round, SALT III, with both sides agreeing they must deal with deep cuts in their force levels.

In other matters, Biden said, he met privately with senior Foreign Ministry official Georgyi Kornyenko to discuss last weekend's Kennedy Airport dispute on Bolsoi ballerina Ludmilla Vlasova. The two men disagreed sharply, he said, on the handling of the situation, but the disagreement did not affect subsequent talks.

The senators also gave Kornyenko a letter they signed to be delivered to Brezhnev raising 79 human rights cases with the Soviet leader. Kornyenko "accepted the letter graciously," Biden said.