A French plan to lend Iraq five attack jets equipped with Exocet missiles is causing growing apprehension in Washington, where officials say they fear Iraq's long-running war with Iran could spread to the entire Persian Gulf region if the planes attack Iranian oil ports or tankers heading for them.

The Reagan administration, through diplomatic channels, has made what officials here describe as "polite inquiries" to the French foreign ministry in an effort to learn more about the plan and what the French expect to result from the unusual arms transfer.

Officials here said there have been some discussions with the French but no detailed response and that Washington has no feel for French intentions.

In many ways, the French dilemma reflects the kind of problem that can crop up for any major arms-supplying nation. Conceived as a gambit to end the war, the plane transfer could escalate it, Reagan administration officials say.

The issue is unusually sensitive here, in Paris and among countries in the oil-rich Persian Gulf because the planes are Super-Etendard navy jets fitted with the highly lethal and extremely accurate Exocet air-to-surface missiles. The Etendard-Exocet combination was used by Argentine pilots last year to sink two British ships in the Falkland Islands war.

Iraqi pilots are still being trained in France to fly the Super-Etendards. U.S. officials speculate that it may be a few more months before the transfer could take place.

Officials in the White House and the State Department said that there has been no confrontation with France over the matter nor any pressure applied on the government of President Francois Mitterrand.

But one official said the "whole idea of sinking a tanker in the gulf is regarded very seriously by us. We would like to see less activity of the kind that could cause this to happen, and providing these things planes is not particularly helpful. So this is a matter of real concern, and people are focusing on the implications of the Super-Etendards."

However, the official said, the French "will do what they want to do."

The main focus of study here is what the United States would do if Iraq used the planes to attack Iranian oil-loading facilities or tankers heading into Iranian ports, such as Khark Island and Bandar Abbas, and if Iran retaliated by trying to block tankers from using the gulf. That could seal off Saudi Arabia, Kuwait and the United Arab Emirates, which are major oil suppliers to the West and which help Iraq financially.

The French deal with Iraq, reportedly worked out in May, was revealed in June by the French newspaper Le Monde. The French government has never confirmed the arrangement, but U.S. officials said that they have no doubt it was made.

A few weeks after the arrangement was publicized, Iran announced that if France provides the planes to Iraq, Iran "will destroy the security of the Persian Gulf" and "make it unsafe for even one ship to enter or exit."

Although the Iranian armed forces have been gravely weakened by the three-year war, U.S. officials said Iran probably still has enough American-built aircraft, torpedo boats and mines to halt shipping in the gulf, if such moves were unopposed.

That raises the question of whether the French fleet in the nearby Indian Ocean would act to keep the gulf open and whether the French could succeed without U.S. help.

It is this potential for escalation that is causing concern here. But that same factor as well as new developments lead some administration officials to speculate that the deal may not go through.

These officials said that when the deal was first worked out, it probably made sense. The war was stalemated, and both sides had suffered enormous losses. Iraq, a much smaller country than Iran and less able to sustain a long war of attrition, had been anxious to negotiate a settlement. There were hints that Iran might agree to a settlement.

The threat posed by the Super-Etendards, it was reasoned, might give Iran extra incentive to negotiate. The Iraqi air force, the U.S. specialists said, has not been particularly effective in the war. Its pilots, to avoid Iranian air defenses, tend to drop their bombs short of the target.

As a result, the Iranian oil facilities have escaped serious damage. Iran continues to do a thriving oil-export business, while Iraq's oil trade has slackened sharply, which gravely depleted its revenues and cash reserves, thus hurting its ability to pay for the war. Loss of their oil facilities would put the Iranians in the same financial straits as the Iraqis.

Iraq has had Exocet missiles for a long time. But it can only launch them from helicopters, which do not have enough range to reach key Iranian targets and which are vulnerable to anti-aircraft fire. Iraq has other French-built planes, but only the Etendard makes really effective use of the Exocet missile, officials said.

The Etendard-Exocet combination, in the hands of well-trained pilots, could knock out Iranian oil facilities, U.S. officials said. Although the progress of the war thus far suggests that neither side is especially skillful militarily, the officials said, a successful Iraqi air attack is not impossible.

The Iranian reaction to news of the French-Iraqi deal was not to move toward negotiations but to threaten to shut down the entire gulf. And now French forces are engaged in Chad, attempting to prevent further inroads by rebels backed by Libya.

It is these two new factors that lead some U.S. officials to believe that France may have second thoughts about delivering the Etendards to Iraq.

In the first instance, the threat to the West's oil is at stake. In the second, Mitterrand could get into more political trouble at home by appearing to involve France militarily in too many places. The Super-Etendards are no longer in production, and France would have to lend planes from its stockpile of about 60.

In addition, France has replaced the Soviet Union as Iraq's major arms supplier and is owed some $5 billion by Iraq for weapons and for numerous projects being developed by French contractors. It also would like to see the war end so that Iraq can pay its debts and French workers can keep on working there. Whether the new weapons would help end the war or escalate it is the crucial question.

The moderate Arab oil states fear an Iraqi collapse, which would strengthen Iran's Ayatollah Ruhollah Khomeini and his militant Islamic fundamentalism. Yet, they also fear that the new weapons would provoke a desperate move by Iran.

The United States, which officially remains neutral between the warring parties, would not like to see an Iranian victory but is wary that an Iraqi escalation could jeopardize larger interests.

Aside from its war with Iraq to the west, Iran is facing growing strains with the Soviet Union to the north. In May, Tehran expelled 18 Soviet diplomats, charging them with interference in internal affairs, and dissolved the Iranian Tudeh (communist) Party.

Moscow responded with some harsh verbal blasts at the Khomeini regime. In addition, sources here said that the Soviets have also virtually brought to a halt railroad traffic across the Soviet-Iranian border, backing up hundreds of railroad cars on the Soviet side. This disrupts trade between the two, the sources said, and hampers Iran's role as a transit route for goods from Europe to other Middle Eastern locations.