Long before their forces shot down an Iranian civilian jet over the Persian Gulf, U.S. Navy officers said their constant fear was making a mistake in a war zone bustling with civilian activity.

Iran said all 290 people aboard the Iran Air jet were killed Sunday when the U.S. guided missile cruiser Vincennes fired at it during a routine flight from Iran to Dubai in the United Arab Emirates.While not officially at war, U.S. sailors are in a combat environment where ships remain at a high state of alert for weeks on end and weapons are tested every few days.

"There are many threats out here, and maybe the worst one is the possibility of somebody making a mistake," one ship's commander said. "In a confined area like the gulf, and with the weapons on hand, the potential for disaster is incredible."

Navy personnel reacted with disbelief after servicemen aboard the Vincennes, one of their most sophisticated warships, mistook an Iranian civilian jet for a warplane and shot it down.

"Basically, they're beating their heads against the walls trying to figure this out," said a source who spoke on condition of anonymity. "Some people are even wondering the worst - could they (he Iranians) have done this on purpose?"

The source said that in the high-tech command information center of the Vincennes all indicators showed that the plane was an F-14 jet fighter that was descending on the ship from six to eight miles out.

The decision by the Vincennes' skipper, Capt. Will C. Rogers III, to fire his SM-2 Standard anti-aircraft missiles was ultimately based on one indicator, the IFF, or "Identify Friend or Foe," which tells radar operators what kind of aircraft is approaching and what it appears to be doing.

"That plane did not answer calls on the radio, and it was flying an attack profile toward the Vincennes. The captain did what he should have done under the circumstances," said the source.

Rear Adm. R.G. Zeller, who returned to the United States on Sunday from six months in the Persian Gulf, said U.S. sailors constantly had to be on the lookout for civilian shipping and aircraft.

During a battle between U.S. and Iranian forces on April 18, "commercial airliners continued to fly as though they were oblivious to what was going on around them, and I think they were," he said in an interview in San Diego. "I don't think they got the word. You have to be very, very careful."

"It's not a war, in the sense that we think of wars where the land and sky is filled with combatants," he said. "It's a Third World kind of thing where you have innocents and combatants and you have to stay extremely careful."

The Vincennes is the first of the billion-dollar, state-of-the-art "Aegis" cruisers assigned to duty in the gulf.

The 9,600-ton, San Diego-based warship arrived in the gulf May 27 and has been assigned since then to anti-aircrart surveillance patrol in the southern gulf, at the edge of the Strait of Hormuz.

The ship's Aegis system is a computer-run "phased array" radar system linked to the missile batteries. The ship, designed for fleet air defense, can see vast distances with its radar and conduct dozens of surveillance and firing missions simultaneously.

U.S. officials have said the Vincennes' multiple task capability could ease the burden on other ships serving in the gulf and give the Navy a far more potent weapon that it has had in the waterway.

From the American standpoint, concern about mistakes stems mainly from the experience of the U.S. missile frigate Stark, which lost 37 crew members and nearly sank after it was mistakenly attacked by an Iraqi F-1 Mirage on May 17, 1987.

U.S. officers and sailors say that while they don't dwell on the Stark incident, it is never far from their minds and has led to major changes in shipboard fire and damage control procedures that are practiced on a regular basis.

The U.S. force in the gulf region, known as the Joint Task Force Middle East, is about 29 ships overall, including the carrier Forrestal and its battle group in the Arabian Sea, and some 16 ships usually in the gulf.

The gulf-based Middle East Force, part of the joint task force, has been escorting 11 U.S.-flagged Kuwaiti tankers since July last year. It includes missile cruisers, missile frigates, destroyers, an amphibious landing ship with 400 Marines on board, and six minesweepers.

Commander of both forces, aboard the flagship Coronado based at Bahrain, is Rear Adm. Anthony A. Less.