The Reagan administration may soon change its convoy escort procedures in the Persian Gulf in what would be the first major response to last month's cease-fire in the Iran-Iraq war, defense officials said Friday.

Under a new plan, U.S.-flag tankers plying the waterway would be kept within a "defensive zone" by American warships but would not be accompanied every step of the way by a Navy vessel as is now the case, the sources said.That change in procedure would allow a slight reduction in the number of warships deployed to the Persian Gulf - probably one fewer ship - but would increase the flexibility of the task force commander and allow the Navy's ships to operate at a lesser alert level, the officials said.

The sources, who agreed to discuss the matter only if not identified, said the change in procedure had been tentatively approved by the Pentagon and White House, but would not be ordered until allies with warships in the region and certain Mideast countries were consulted.

The Defense Department declined to discuss details of the plan on Friday but acknowledged in a statement:

"The administration is considering additional steps which . . . (could be described as) modest modifications of our present method of providing protection to U.S.-flagged shipping in the gulf.

"The modifications currently under consideration do not involve any significant reduction in force levels. We are continuing to consult with our friends and allies."

The United States has 26 Navy ships assigned to its Joint Task Force Middle East, including 17 inside the Persian Gulf itself. As of Friday, the Navy had conducted 85 convoy operations since the summer of 1987, when the escorts began.

With the exception of the departure of one cruiser, the USS Vincennes, the naval force has not changed in size despite last month's end to hostilities between Iran and Iraq.

Defense Secretary Frank C. Carlucci has said repeatedly the United States will not reduce its military presence in the region until it is absolutely convinced the cease-fire will hold and that American merchant ships aren't threatened.

According to the sources, Carlucci has approved the idea of "providing flexibility" to the Navy on convoys.

"The best way to describe it is in basketball terms, by saying you're going from a man-to-man defense to a zone defense," said one official.

"You keep your warships nearby, always close enough to act, but drop the continuous, side-by-side presence," added another source.

"The Navy vessels deploy in what amounts to a picket line up and down the gulf, ready to pick up a merchant as it moves within a particular range," a source said.

The sources said the plan called for the Navy to directly escort merchants through the narrow Strait of Hormuz, the sole entrance to the Persian Gulf, but to use the picket-line approach for the remainder of the trip.

"This is the first important response to the cease-fire," said one source. "If this works out OK, then we can consider additional steps."

The United States mounted its escort operations in 1987 after President Reagan approved a request by Kuwait to reflag 11 of its tankers under the American flag. Kuwait, a key ally of Iraq in the 8-year-old war, had seen its ships become a special target of Iranian forces and wanted to obtain U.S. military protection.