Reagan administration officials, breaking with their normal caution in Middle East forecasting, say they see an unexpected chance now for a breakthrough in long-stalemated diplomatic efforts to bring Iran to the negotiating table and perhaps end the seven-year-long Persian Gulf war.

These officials say that Iran's decision to receive U.N. Secretary General Javier Perez de Cuellar to discuss "implementation" of a Security Council cease-fire resolution represents something of a watershed in Iranian diplomacy -- if not yet a change in its war objectives -- that offers the first opportunity to engage Tehran directly on a war settlement.

They attribute this new Iranian attitude mostly to Iran's growing isolation, caused largely by unusual big-power cooperation in applying pressure on Tehran to halt hostilites. They also acknowledge that Iran's diplomatic tactics could easily change again overnight, or a reescalation of the fighting could destroy the chance for a breakthrough.

Ironically, it is the unpredictable actions of Iraq -- the country that would benefit most if U.S. efforts to end the war succeed -- that are now a source of great concern among some administration officials. They fear that Iraq's announced policy to continue attacking Iranian oil facilities in the gulf until a final political settlement is reached could provoke Iran to break off peace talks.

The Iranians, who have traditionally insisted on numerous difficult conditions before they would join in peace talks, have now reduced their demands to one: that the United Nations condemn Iraq as the initial aggressor in the war.

The U.N. cease-fire resolution of July 20 provides for establishing an "impartial body" to investigate who was responsible for the war. Iraq has said it is ready to let the International Court of Justice decide this issue.

Iranian President Ali Khamenei yesterday told Perez de Cuellar, who left for Baghdad after two days of talks in Tehran, that Iran's cooperation in a search for a war settlement still depends on the United Nations finding "a formula" for punishing Iraq as the aggressor.

His and other Iranian statements about the U.N. secretary general's visit left unclear whether any real progress had been made to substantiate the administration's optimism about chances for a breakthough.

In the past, Iran has refused even to discuss U.N. cease-fire appeals because it regarded them as biased against it and the work of the "Great Satan," the United States. This time it is faced with a resolution that all five permanent Security Council members -- the United States, the Soviet Union, China, Britain and France -- have supported.

"I think there is a possibility at least that the Iranians are realizing they have painted themselves into a very tight corner," said one senior administration official.

He and other U.S. Middle East policymakers are hoping the current U.N.-led peace bid will entrap Iran in a peace process that even hard-line Iranian leaders will have to accept to keep up a degree of international support -- and continuation of arms supplies.

Iraq, which seemingly has the most to gain from a cease-fire, has done the most recently to sour the atmosphere for Perez de Cuellar's long-postponed peace mission by suddenly resuming the "tanker war" in the gulf, according to these officials.

"We think the Iraqi move was self-defeating for what they want to get done," said the same senior official.

He said Iraq had lost "the high ground" with Iran by resuming its attacks on Aug. 29, after a six-week lull, and "blurred" the international perception that Tehran was more at fault than Baghdad for the continuing war.

Iraq has repeatedly insisted on Iran's acceptance of a total cease-fire -- on land, sea and in the air -- and of an overall settlement to the conflict before it will end its attacks on Iranian gulf oil facilities and tankers.

The U.N. measure calls not only for a halt to hostilities but the withdrawal of the two countries' forces to internationally recognized borders and a negotiated political settlement to the conflict.

The likely immediate prospect, U.S. officials fear, is a kind of gray diplomatic situation of no real war nor peace that Iraq will find intolerable, one in which Iran will continue to avoid a clear-cut total acceptance or rejection of the U.N. cease-fire resolution but leave open the door for further negotiations.

U.S. officials concede that such an Iranian diplomatic ploy could also derail the administration's plan to press immediately for a U.N. arms embargo on Iran in the absence of its clear acceptance of the cease-fire resolution during Perez de Cuellar's Tehran visit.

They say they fear that if the U.N. secretary general, on the basis of his consultations with Tehran and Baghdad, asks for more time to allow diplomacy a chance, both the Soviet Union and China will back him. In this case, the United States would have serious difficulty in getting the five permanent U.N. Security Council members to unanimously support a resolution calling for an arms embargo on Iran.

The senior administration official indicated there is already concernthat the Soviet Union and China will abstain on a vote to impose an arms embargo in any case.

Administration officials, who readily admit U.S. intelligence on internal Iranian politics is poor, say they have received no reports that Iran's unpredictable and often volatile leadership has suddenly abandoned its main war goal -- the overthrow of the Iraqi regime under President Saddam Hussein.

"There is a shift in Iranian diplomacy," remarked another U.S. official, "but is there a shift in objectives? I doubt it."

The general U.S. government assessment is still that until Iran's fiery religious leader, Ayatollah Ruhollah Khomeini, dies the likelihood of a change in Iranian war objectives remains slim.

Still, U.S. officials are hoping to take advantage of the current shift in Iranian diplomacy -- which has included promises of "full cooperation" with the U.N. Security Council -- to begin the process of entrapping Iran in a peace process.

They say the diplomatic shift has included a change in the tone of messages Tehran has been relaying to Washington through third parties, including the Swiss and Japanese, which they describe as "more measured" recently. The Iranians, they say, have been telling the administration they neither want, nor seek, a confrontation with the United States in the gulf.

Yesterday, Secretary of State George P. Shultz disclosed that Iran has been sending the United States "many messages" through Switzerland and Algeria and said it was "probably a good thing if representatives of the two countries directly communicate with each other . . . to avoid miscalculation."

One recent turning point for the Iranians, according to administration analysts, came when Iran began mining international waters, even outside the Strait of Hormuz.

The mining prompted Western European nations, which initially spurned U.S. entreaties for support, to change policy and send minesweepers and warships to the gulf. The Soviets also sent new ships into the area.

A total of about 75 warships and other naval vessels -- from the United States, Britain, France, Italy, the Netherlands, and the Soviet Union -- are being assembled to protect shipping and sweep the gulf for mines -- an outcome the Iranians did not seek.

Another serious Iranian miscalculation, according to U.S. officials, has been the recent firing of at least three Chinese-made long-range missiles, probably Silkworms, from Iranian-occupied territory in Iraq to Kuwaiti territory.

The Kuwaitis artfully succeeded in embarrassing the Chinese publicly, they said, by calling in the ambassadors of the five U.N. permanent Security Council member countries and showing them fragments from the missile on which Chinese markings were clearly visible.

"No one had the rudeness or spirit to ask for a translation but I assume it said, 'This was not sent by the People's Republic of China,' " quipped one U.S. official.

The administration has been pressing Beijing "very hard" to halt arms shipment to Iran, according to U.S. officials.

According to U.S. estimates, China ranked first, with $400 million in sales, among Iran's foreign providers during the first seven months of this year.