The White House said Monday that American warships will no longer form convoys to protect U.S.-flagged tankers in the Persian Gulf but will use instead a zone system to guard the vessels.

White House spokesman Marlin Fitzwater said the gulf states involved were consulted last week through military and diplomatic channels. He said President Reagan made the decision before leaving to address the U.N. General Assembly.Fitzwater said it will take several days to begin the new policy and that the dates for its start probably will not be announced.

"After reviewing the current cease-fire in the Persian Gulf and consulting with allies and friends in the region, the president decided to modify our present method for providing protection for U.S.-flagged shipping in the gulf," Fitzwater said.

"The modification entails a change from a tight escort procedure to an `accompany' regime," he added. "In an accompany regime, U.S. forces will be positioned such that they can take appropriate action if U.S.-flagged ships are directly threatened," he said.

Fitzwater said there have been no attacks on U.S.-flagged shipping since the Aug. 20 cease-fire in the Iran-Iraq War. He said 180 ships have been escorted through the gulf waters since July 1987.

As for any kind of signals from Iran on normalizing relations, Fitzwater said, "We've always said we're willing to talk to authorized Iranian officials." But he said there have been no new signals from Iran and no word on the fate of American hostages being held in Lebanon.