Reagan administration officials said yesterday that they hope Iran and Iraq will agree to a temporary truce in their seven-year-long war, allowing U.N. Secretary General Javier Perez de Cuellar a chance to work out a permanent cease-fire during his proposed visit to Tehran next week.

The five permanent members of the U.N. Security Council, who agreed Wednesday that Perez de Cuellar should accept an Iranian invitation to visit Tehran, met with him yesterday to discuss terms for his talks with Iranian officials.

Later, the secretary general's spokesman, Francois Guiliani, said all 15 council members will meet this morning to approve formally his trip to Tehran and Baghdad as well as his negotiating instructions.

In light of the developments, State Department spokesman Charles E. Redman said yesterday the United States has dropped its demand for a "definitive" Iranian reply by this weekend to the U.N. cease-fire call of July 20 and will delay seeking a mandatory arms embargo on Tehran. On Tuesday, the department said the United States would begin work on such a draft U.N. resolution early next week if Iran did not give a clear-cut reply.

A senior official said U.S. officials are optimistic that Iran and Iraq will accept a "cooling off" period in the war in light of Perez de Cuellar's visit to them Sept. 10 to 13. The official said that a cease-fire is "not a precondition of the trip by the secretary general, but it is something we'd like very much."

In Washington, a State Department official said he expects Perez de Cuellar to "suggest" that both sides "observe a cease-fire on land, sea and in the air" to facilitate his trip to Tehran.

Administration officials said they hope a temporary halt in hostilities, if both sides agree to it, could be parlayed into a permanent cease-fire.

Redman said informal consultations the past two days among the five permanent U.N. Security Council members -- the United States, Soviet Union, China, Britain and France -- were "serious and productive." He said it was "particuarly important" that the five had agreed that Perez de Cuellar should limit his discussions in Tehran to "the basic issues" concerning an Iranian reply to the July 20 U.N. cease-fire resolution.

A senior Iranian foreign ministry official, Mohammed Jafar Mahallati, replied Wednesday night to questions from Perez de Cuellar regarding the Iranian attitude toward the resolution. U.N. spokesmen refused to say whether Iran had made its position clear.

An Iranian spokesman in New York said Mahallati had given an "unconditional positive response" to the questions posed by the secretary general and that he saw no obstacles to his visit to Tehran. However, the U.S. ambassador to the United Nations, Vernon Walters, said Iran still had not committed itself to accepting the cease-fire resolution.

An administration official said that the administration had decided not to press for a U.N. resolution calling for an arms embargo against Iran until after Perez de Cuellar reports to the United Nations following his Tehran visit.

"Iraq understands we'd like a renewal of a cease-fire in the tanker war," said a senior official. "Iran understands that we're not going to press for an arms embargo until the U.N. has exhausted other remedies.

"We understand the Iraqi desire for an end to the war and the Iranian desire to be able to move its oil through the gulf," he added.

The official would not say on what basis these understandings were reached. He declined to discuss whether secret negotiations between the United States and Iran were taking place outside the forum of the United Nations.

Dan Howard, a deputy White House spokesman, would say only that the United States was "encouraged" that the five permanent members of the Security Council were working together to draft instructions for Perez de Cuellar to take to Iraq and Iran.

Officials said that while final instructions remained to be worked out, the U.S. view is that the Perez de Cuellar will have narrow latitude to change the wording of the July 20 U.N. resolution but broad latitude to work out conditions for its implementation. The resolution calls for a comprehensive cease-fire, the withdrawal of the two nations' forces to their internationally recognized borders and a negotiated settlement of the conflict.

Iran has been insisting that Iraq, which started the war in the fall of 1980, be branded the "aggressor" before Iran will agree to a cease-fire. The U.S. position is that a cease-fire is the starting point for ending the war.

Officials here said that Iraq has been assured in diplomatic exchanges that the United States would oppose branding Iraq as an aggressor. But they said that U.S. diplomats have strongly urged Iraq to resume the unilateral cease-fire that it had observed for 45 days before resuming attacks on Iranian oil tankers last week.