United States and Soviet negotiators apparently reached a formula for a new strategic arms limitation treaty (SALT) tonight but had to refer their proposed settlement to Moscow and Washington for final approval.

A formula for agreement on the last two major issues was reportedly worked out in 3 1/2 hours of meetings late today between Secretary of State Cyrus Vance and Soviet Foreign Minister Andrei Gromyko.

All indications were that U.S. approval was not in doubt, but sources cautioned, however, that the settlement of outstanding issues cannot be considered finally resolved before Kremlin approval, which is not assured.

White House Press Secretary Jody Powell phoned The Washington Post twice during the nightto deny emphatically that a tentative agreement had been reached, saying "at least one or two issues" remained unresolved. Powell based his statement on President Carter's phone conversation with Vance. Other senior U.S. officials indicated that an agreement was in hand but expressed nervousness that the Soviet leadership in Moscow might not sustain Gromyko's tentative approval.

[The administration appeared concerned that premature press reports of a settlement could derail the delicate talks an annoy key members of the Senate, which would have to approve a treaty. It was learned that Vance instructed adminstration lobbyists not to discuss the results of the Geneva negotations until Moscow gives its final response.]

One U.S. official said it might take several days before a definitive Soviet response is received but the expectations here were that it would be positive.

The ministers also discussed a summit meeting between Carter and Soviet President Leonid Brezhnev. Further discussion of the summit is expected at another Vance-gromyko session Saturday morning, but no summit announcement is likely from here.

State Department spokesman Hodding Carter announced after today's meetings that "we are close to the end of the road" and that Vance and Gromyko would discuss the progress they have made when they meet reporters following their last meeting here Saturday morning.

"It looks good" for completion of the long-awaited SALT II agreement, said another U.S. source.

The final two issues in the six years of bargaining over a new SALT pact, according to informed sources, involve the Soviet practice of transmitting some missile test data in code, so that it cannot be checked by U.S. monitoring systems, and the application of SALT restrictions to some air-launched cruise missiles which are not equipped with nuclear warheads.

U.S. SALT negotiator Victor Karpov drafted compromise language on these issues prior to the climactic meeting late this afternoon at the Soviet mission here. Their meeting ran overtime and reportedly did not produce agreement.

Vance and Gromyko, accompanied by a small working group of their senior SALT experts and ambassadors in one another's capitals, discussed the compromises for two hours starting about 5 p.m.

Following that session the two foreign ministers met again for 90 minutes with an even smaller group composed of the U.S. ambassador to Moscow, Malcolm Toon, Soviet Deputy Foreign Minister Georgi Kornienko and the Soviet ambassador to Washington, Anatoliy Dobrynin.

Spokesman Carter said the later meeting involved not only SALT but "bilateral and international affairs." He did not discourage speculation that the summit meeting to sign the SALT agreement had been among the "bilateral" topics.

SALT will be the first subject on Saturday morning's agenda followed by "other matters," Carter said.

All indications here were that several questions remain to be resolved about the Carter-Brezhnev summit. Carter has said he would like to play host to Brezhnev in Washington in mid-January, but another official said the Soviets may decide to put off the Brezhnev trip until after U.S. visit of Chinese Vice Premier Teng Hsiao-ping, which is scheduled to begin Jan. 29.

If Moscow approves the agreed terms of SALT II, it will required several weeks for members of the two SALT delegations in Geneva to iron out secondary details and put the draft treaty in the final form.

The formal resolution of the problems concerning the Soviet Backfire bomber is expected to be reserved for statements by each side at the summit. The substance of the assurances to be made by the Soviet side, howver, are reported to have been worked out. The Backfire is not counted under SALT, in keeping with Soviet insistence that it is a medium-range bomber which is not a strategic weapon.

Details of the compromises on the missile data and cruise missile issues were not available here tonight.

There were reports, however, that the Soviet Union was ready to ban any type of encoding of missile test data which impedes SALT verification.

Negotiators on both sides are believed to have addressed the question of what this means in practice.

The issue of defining air-launched cruise missiles as covered by the SALT pact, results from a U.S. effort to protect the options of developing future pilotless drones of this type which carry cameras or conventional weapons rather than nuclear warheads.

The United States proposed to design externally observable characteristics to distinguish these cruise missiles from the nuclear-armed variety, but the Soviets objected that the difference in the warheads in the nose of the missiles would not be verifiable.

Cruise missile issues are particularly sensitive to the West Germans and other West European allies, who consider them of great potential value.

Vance is scheduled to leave Geneva for Brussels Saturday afternoon for talks with the Egyptian prime minister and Israeli Foreign Minister Moshe Dayan in an effort to revive the Middle East Negotiations.