High in the hazy skies over the Strait of Hormuz, a U.S. fighter pilot last Saturday did what his counterparts on a U.S. warship in the Persian Gulf failed to do three months ago: He took the split-second leap into military combat.

Sighting on his radar what appeared to be an Iranian warplane approaching at speeds that turn miles into milliseconds, the Navy F14 Tomcat pilot fired two missiles at what he judged to be a hostile threat.

Almost simultaneously, the radar "blip" signifying an intruder aircraft showed that it banked sharply, turned and disappeared, dodging the radar-guided Sparrow missiles, according to military accounts.

"It was the USS Stark revisited," one military official said.

In the tension-charged Persian Gulf, where the crews of U.S. warships stand at battle stations alert to escort reflagged Kuwaiti tankers through the Strait of Hormuz, the May 17 missile attack on the frigate Stark that killed 37 sailors and wounded 21 is exerting heavy influence over the actions of military officers, according to well-informed Pentagon officials.

Last Saturday morning the parallels were striking, said sources familiar with the missile-firing incident, which administration briefers did not disclose until three days after it occurred and still refuse to confirm on the record.

But in reconstructing the incident, sources say that just hours after a Navy-escorted convoy of reflagged Kuwaiti oil tankers began plowing through the calm waters of the Strait of Hormuz, U.S. officials noticed what they believe was an Iranian U.S.-built F4 Phantom fighter tailing one of the Navy's P3 surveillance planes flying ahead of the escorting mission, according to sources.

U.S. aircrews warned the Iranian plane away. But it did not respond. "Nothing . . . it kept coming, coming, coming," one Pentagon official said.

In the strait below, a radioman aboard a sophisticated American air-defense Aegis cruiser ordered the intruding jet to change course. Still no response.

Then, a formation of F14 fighters accompanying the escorting mission streaked across the sky, ready to protect the lumbering P3, according to the scenario sketched by administration officials.

The twin-engine, swing-wing fighters, from the aircraft carrier Constellation steaming in the Arabian Sea outside the gulf, are the Navy's hottest combat aircraft.

The P3 was never "in imminent physical danger," according to officials. But the opposing planes drew closer and closer, and in one fraction of a second, the U.S. jets entered the potential missile range of their adversary.

In a moment, an American pilot fired two Sparrow air-to-air missiles. Guided by radar from the launching jet and its own equipment, the rockets streaked ahead, but both missed their targets.

It was all over in a matter of seconds.

The Americans never visually sighted the presumed intruder plane.

Now, four days later, administration and military officials say they are not certain what type of plane caught the attention of U.S. sensors, or whether any enemy airplane even existed.

"Nobody was going to take any chances," one administration official said.

On a night three months ago, the crew of the USS Stark, one of the U.S. warships stationed in the Persian Gulf, took a chance.

The ship's electronic monitors as well as a nearby surveillance plane spotted an aircraft flying erratically toward the Stark.

Identified as an Iraqi warplane, it should have been a "friendly." But it continued to approach and after long minutes, the ship's radioman warned the plane to turn away.

But it was too late.

By the time the Stark's officers realized the danger, two Exocet missiles were homing in on the ship, and there was no time left to activate weapons or defenses.

In the aftermath of that incident, Defense Secretary Caspar W. Weinberger stressed anew the "rules of engagement" for firing at potentially hostile threats in the gulf region, giving military officers more assurances of their ability to defend themselves.

"The best chance to avoid a repeat of the Stark incident is to be absolutely clear we mean to retaliate, we mean to fire on unidentified planes," said House Armed Services Committee Chairman Les Aspin (D-Wis.) yesterday. "The way to save lives is to be very aggressive."

Officials, who said they are still trying to piece together the events, have not ruled out that the Saturday incident could have been a false alarm. An earlier false alarm occurred in the same area of the strait when Navy planes detected what they first believed to be activity at a previously unknown Silkworm antiship missile launching site on the Iranian side of the waterway. After a flurry of activity, it was ruled a false alarm.

In the fighter incident, officials said, sophisticated equipment in U.S. planes and ships could have displayed a false warning signal about an approaching aircraft.

In addition, officials said, an airplane in a different vicinity could have used its electronic countermeasures to give off a false location to the U.S. search equipment.

"We may never know for sure what kind of a plane it was, or if there was a plane," one administration official said.

Sources said the plane was tentatively identified as an F4 because of its perceived speed and maneuvers.