Saudi Arabia has agreed "in principle" to use its recently purchased U.S. radar surveillance aircraft to help protect U.S-escorted Kuwaiti tankers against possible Iranian attacks as they pass through the lower Persian Gulf, the State Department announced yesterday.

Department spokeswoman Phyllis Oakley said the United States and Saudi Arabia have agreed "in principle on a cooperative arrangement" for a southern orbit flown by Saudi airborne warning and control system (AWACS) aircraft.

"We are continuing to work out the modalities of such an arrangement," she said.

The agreement, if implemented as planned, would extend the use of Saudi air power for the first time beyond Saudi territorial waters in the gulf, a commitment the Saudis have heretofore refused to make.

The Saudis informed the U.S. government of their agreement to aid in the lower gulf last week and the message was relayed through normal diplomatic channels to Washington. The Saudi ambassador to the United States, Prince Bandar Bin Sultan, Friday delivered a letter from King Fahd to President Reagan but it did not concern the Saudi AWACS decision, according to administration sources.

U.S. officials had already disclosed the Saudi accord to aid the United States in providing air protection for the 11 Kuwaiti oil tankers that are being placed under U.S. flag protection. But it was not previously known that Saudi, rather than American, AWACS were to be used in the lower gulf.

The southern orbit, to be flown by Saudi AWACS protected by Saudi F15s, supplements the northern orbit now being flown by four U.S.-staffed AWACS. These aircraft operate within Saudi territory but provide intelligence on ship movements in central and northern sectors of the gulf. Saudi Arabia has also been providing air protection for the American AWACS with its F15 jet fighters purchased from the United States.

The first American AWACS was not previously scheduled to leave the kingdom before the end of this year as the Saudis progressively learn to fly their own aircraft. This schedule may now be delayed, however, as more AWACS are needed to maintain two surveillance orbits that cover nearly the entire gulf.

U.S. officials explained that unlike the northern orbit, which is a 24-hour operation, the southern one may only be needed periodically when U.S. Navy ships escort the reflagged Kuwaiti oil tankers in convoys from the Strait of Hormuz to Kuwait.

It was not immediately clear how the Saudis will manage to carry out their expanded air-defense mission in the lower gulf.