The U.S. Navy has begun cutting back its Persian Gulf force by dropping plans to send another sophisticated Aegis cruiser to the region.

Although other deployments may go ahead as planned, U.S. military sources say the move indicates that senior American officials are confident that an Aug. 20 cease-fire between Iraq and Iran will succeed, clearing the way for a promised reduction of the American naval commitment here.By a process of attrition, the 27-ship force could be trimmed considerably by the end of October, although probably not to the level of "three, four or five ships" mentioned by Defense Secretary Frank C. Carlucci in a statement last week, the sources said.

To bring it to that size, about where it was last year when the American naval buildup began, would mean removing the 11-ship carrier battle group in the Arabian Sea and almost two-thirds of the 16 ships now in the gulf.

Six of the ships in the gulf are mine sweepers, whose presence may be required indefinitely, according to senior U.S. officers. They say a "couple of hundred" mines are still in the waterway.

First word of the reduction came when Navy sources disclosed that Friday's scheduled departure of the Aegis-type cruiser Mobile Bay from its home port at Mayport, Fla., was being "temporarily postponed."

Pentagon spokesmen said the 9,600-ton Mobile Bay, which had been marked to replace its sister ship Vincennes in the gulf region, would remain in the United States while U.S. officials re-evaluated the situation in the waterway.

Cmdr. Mel Sundin, a Pentagon spokesman, said a final decision on the Mobile Bay will depend on an ongoing assessment of "what level of forces are needed to ensure continued and unimpeded operations in the gulf."

The military sources, speaking on condition of anonymity, said that meant the ship would head for the gulf only if the U.N.-sponsored cease-fire collapsed and a new flareup endangered the Navy's escort operations.

Cmdr. Mark Baker, a Navy spokesman at the Pentagon, said Saturday the Navy had no change in plans to deploy four other warships scheduled to leave the east coast for gulf duty this week. He was clarifying a Friday news report that quoted a Pentagon statement as saying a decision was not made on the deployment.

The ships include the missile cruiser Josephus Daniels, based at Norfolk, Va., and the missile frigates Taylor, Aubrey Fitch and Doyle. The Taylor is based at Charleston, S.C., and the other two at Mayport.

Barring a change of orders, they would replace other ships now serving in the gulf. The present flotilla includes six vessels due to leave by late September.

The Vincennes, which arrrived in the region May 22, will depart for San Diego early next month, U.S. officials said. Some observers speculated that it might leave sooner because of the uproar over its attack on an Iranian jetliner on July 3, killing all 290 aboard. The Navy said the crew mistook the jet for an attacking Iranian fighter.

A U.S. officer said, however, that ship movements are always flexible within a certain range, and the Vincennes' schedule would give time to determine whether the cease-fire would work.