UNITED NATIONS, SEPT. 24 -- Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze agreed today to defer temporarily the U.S.-backed drive for a worldwide arms embargo against Iran while U.N. diplomats explore new possibilities for a voluntary Iranian cease-fire in its war against Iraq, according to U.S. sources.

The Soviet-American agreement, which is likely to be joined by Britain, France and China in a meeting here Friday, was depicted by diplomats as an effort to "preserve the unity" of these five big powers who are the veto-wielding, permanent members of the U.N. Security Council.

Shultz and Shevardnadze both spoke of the importance of sticking together on the U.N.-related diplomacy as they emerged from their 90-minute session at the U.S. Mission to the United Nations. They gave no details of their meeting, which Shultz called "a very constructive and worthwhile discussion." Shevardnadze said the session produced "agreement in principle" to "preserve the unity among the permanent members of the Security Council."

Shultz also announced that he and Shevardnadze had agreed to meet in Moscow Oct. 22-23 to review arms control progress and other main elements in U.S.-Soviet relations and to "set the precise date" for a fall summit meeting of President Reagan and Soviet leader Mikhail Gorbachev.

Despite hints to the contrary from Moscow, U.S. officials said there is "no alternative" to having the business part of the summit meeting in Washington and that they have heard nothing from Soviet officials suggesting any objection to the nation's capital as the meeting site.

The officials also said they have received no information from Soviet diplomats or other sources suggesting that Gorbachev's seven-week absence from public view is due to serious illness or political difficulty.

The U.S.-Soviet meeting of minds today appears to reflect some accommodation on each side, as well as a new maneuver by Iran on the subject of a cease-fire in the Iran-Iraq war.

U.N. diplomats said that late yesterday senior Iranian officials who accompanied Iranian President Ali Khamenei here presented a new written proposal to aides of U.N. Secretary General Javier Perez de Cueller that included a slight shift in Iran's position.

Iranian authorities previously had told Perez de Cuellar that Iran would accept a de facto, or unannounced, cease-fire in the hostilities in the region, to take effect when a U.N. commission began work on assessing blame for the conflict. An openly announced cease-fire would follow when the commission branded Iraq as the aggressor.

"Exploration" of such a U.N. commission, primarily as a concession to Iran, was one of the elements of a July 20 U.N. Security Council resolution that also ordered both Iran and Iraq to cease firing and return to internationally recognized borders.

The new plan by Iran, sources said, envisages a de facto cease-fire when the U.N. commission of inquiry is named and an acknowledged cease-fire when the same commission, having determined which combatant is the aggressor, enters a "judicial" phase to decide on punishment.

U.S. officials expressed skepticism that Iran's new posture can be further modified into an actual cease-fire on terms acceptable to the international community.

A new element, however, was Soviet endorsement in Shevardnadze's U.N. speech yesterday of a voluntary cease-fire simultaneous with the establishment of the U.N. commision of inquiry.

U.S. officials are depicted as having decided it would be worthwhile to agree to permit Perez de Cueller to explore the Soviet and Iranian proposals further, probably for a limited period of time, before pushing ahead to the arms embargo that Washington seeks.

The official rationale for this is that there has been enough movement toward an Iranian accommodation in diplomatic talks to justify seeking such a voluntary agreement.

If this exploration is unsuccessful, U.S. officials would expect the Soviets to do their part for U.N. Security Council "unity" by moving ahead to the arms embargo.

Shevardnadze, as he left the meeting with Shultz, said both sides agreed to work for "implementation" of the July 20 Security Council demand for an end to the fighting. U.S. officials read that statement as an indication of Soviet willingness to take stronger enforcement measures if the diplomatic exploration suggested by Moscow does not bear fruit.

Neither Shultz nor Shevardnadze mentioned the Soviet's earlier proposal that an international force under the U.N. flag take over the job of protecting shipping in the gulf from the U.S. armada and western European ships now in the area. Special correspondent Michael J. Berlin contributed to this article.