As the United States nears a decision that could once again make it a supplier of arms to Iran, France is positioning itself more strongly behind Iraq in the belief that the Baghdad government will be by far the more powerful regional power after the fighting has ended.

The twin developments have fueled fears among some in France that the Persian Gulf conflict could become one between French and American arms in the hands of the Iraqis and Iranians, respectively.

Some key French officials today privately expressed what they said was their shock over what they believe is U.S. acceptance of the "blackmail" of the swap of the American hostages for U.S. spare parts of Iranian weapons.

French analysts said that the eventual U.S supply of parts for planes and helicopters to Iran can only have the effect of prolonging the war and further contributing to the destruction of Iraq's petroleum production facilities.

These analysts believe, however, that even should Iraqi President Saddam Hussein fall from power, Iraq is far better bet for future usefulness than Iran because Iraq is a highly centralized state that can be counted on to make a vigorous reconstruction effort and resume producing oil for export as soon as the war ends. Iran, on the other hand, according to these analysts, has destroyed its own state structure and must rebuild that before it can do anything else.

Despite its officially proclaimed neutrality in the Persian Gulf war, France has been backing Iraq with increasing openness.

"France is Iraq's only Western friend," one of the highest ranking officials here said last week. Today another top insider said, "Why can't you Americans understand that Iraq is the shield against the Soviets in the gulf? The greatest threat to destabilization in the gulf would be the collapse of Iraq."

A top analyst expressed approval for a statement yesterday by center-left politician Jean-Jacques Servan-Scheiber that Washington is taking the "historical responsibility" of supplying "a modern air force" to Iran to give it unquestioned air superiority.

"The whole Arab world," he said, "will then consider there has been an aggression not only by Iran but also by the United States and, consequently, the West."

While three-quarters of Iraq's equipment is still Soviet, Iraq's most recent and modern equipment is French. France sold Iraq $2 billion worth of arms in 1977 and 1978 alone. Soviet help to Iraq during the war has been minor.

In the face of official no-comments, there have been persistent French press stories that France is going all-out to deliver material to Iraq. But reports that France is about to deliver some of the 60 advanced Mirage F1 jets that Iraq contracted for delivery starting in mid-November seem premature.

"That's a high-visibility item," commented a Western diplomat.

The French are nevertheless understood to be continuing to supply a long list of less dramatic items like helicopter spare parts, antitank rockets, field guns and parts for medium tanks.

France is clearly doing everything it can to prevent the downfall of Saddam Hussein and seems to be looking for ways to help him bring the war to an end before it is too late for his political survival.

U.S. military help to Iran, which is pledged to overthrow him, would undoubtedly complicate French efforts. Top French officials claim that the Soviets are working to eliminate the Iraqi leader and to replace him with someone they could control.

The Paris newspaper Le Monde reported today from Tehran that "Western sources" there say that all 800 Soviet military advisers normally stationed in Baghdad have been called home. But U.S. sources here say they have seen no evidence of that, even though the Soviets have obviously been administering their military assistance to Iraq with an eyedropper.

French officials said that Iraqi Vice Premier Tariq Aziz, who came to see President Valery Giscard d'Estaing right after going to Moscow at the start of the war, is returning to Paris very soon to seek French help in ending the war.

In the French view, the Soviets are now betting on Iran and will work for the preservation of Iranian national unity because the Islamic revolution will be a permanent threat to the moderate pro-Western Arab regimes in the Persian Gulf region.

"The gulf states showed early in the war that they were siding with Iraq, even if they backed off later, and Iran is not going to forgive or forget," said a senior French analyst.

Servan-Schreiber, who appears to be working for a comeback as an ally of Giscard, spoke of a reequipped Iranian Air Force being a threat to Saudi Arabia, Kuwait and the rest of gulf Arab states.

The Soviets are now working to form a new Middle East front combining the radical states and the religious extremists, with Iran, Syria, Libya, South Yemen and, if their current maneuvers succeed, the next Iraqi government as well, the senior analyst contended.

In that effort, the Soviets recently started leaning on Iran's pro-Moscow Communists, the Tudeh Party, to work for the release of the U.S. hostages, he said. The reasoning is that the United States will remain the Islamic revolution's great-power enemy no matter what Washington now does and that the Soviet Union will emerge as the arbiter in the gulf.

The French are prepared to move in elsewhere in the gulf besides Iraq. An agreement was signed yesterday by French Interior Minister Christian Bonnet with Saudi Arabia to train Saudi internal security officers and provide equipment for their forces, functions that used to be performed by the United States.

France has had the inside track for Saudi internal security ever since the French sent a special police antiterrorist unit a year ago to advise the Saudis on how to root out the rebels entrenched in the Grand Mosque at Mecca.

France recently resigned a naval equipment contract with Saudi Arabia for $3.5 billion, and before the Iraqi-Iranian war it was bidding against Italy for a $1.8 billion contract to equip the Iraqi Navy with dozens of ships.

In contrast to its deliveries to Iraq. France has scrupulously followed the U.S.-inspired embargo against Iran, blocking in Cherbourg the delivery of the last three of the 12 high-speed patrol boats Iran had on order.