WASHINGTON (AP) _ Weeks before Kuwaiti oil tankers start carrying American flags, the U.S. Navy escorted an arms-bearing Kuwaiti merchant ship across the Persian Gulf, the Pentagon says.

In an unexpected first step towards expanded activities in the Persian Gulf, four U.S. warships along with P-3 Orion surveillance planes guarded the 16-hour journey of the cargo ship from just outside the Strait of Hormuz to Bahrain, two Pentagon sources said Tuesday. The Pentagon announcement did not give details of the operation.

The ship carried American-made M-60 tanks, howitzer artillery and ammunition that had been sold to Bahrain by the United States, Pentagon officials said. The ship completed its journey Monday night ''without incident,'' the Pentagon said.

The Defense Department said it decided to offer escort protection to the vessel because ''it is important that the United States remain a reliable supplier of defense items to friendly countries.''

''On a case-by-case basis, given the situation in the Persian Gulf, we will escort (such) shipments to friendly non-belligerent countries,'' the Defense statement added.

The Reagan administration had previously announced only plans to provide escort protection to Kuwaiti oil tankers that change their registry to the U.S. flag. That protection is expected to begin sometime early next month, according to Pentagon sources.

The administration had said nothing before Tuesday about using U.S. warships to keep an eye on foreign vessels if they were carrying American-made arms. Pentagon sources said they were not sure whether such protection had been extended previously to a foreign vessel.

The administration has drawn criticism for its planned Persian Gulf strategy on Capitol Hill in the wake of the May 17 attack on the frigate USS Stark. The Navy warship was heavily damaged in an Iraqi air attack that left 37 sailors dead. Both the United States and Iraq have described that attack as an accident.

Although it was the first on an American vessel in the gulf, the Navy's work in the area has become increasingly dangerous because Iran and Iraq - which have been fighting each other for almost seven years - have stepped up attacks on commercial shipping.

The sources identified the Kuwaiti merchant as the Ibn Rashid and said it was met by a Navy warship just outside the Strait of Hormuz, the entrance to the gulf.

''One ship would keep an eye on her for awhile and then hand off to the next one up the gulf,'' said an official who asked not to be identified. ''Everything went smoothly.''

A second source said the Kuwaiti ship had also been shadowed almost continuously by P-3 Orions, long-range planes used by the Navy primarily for submarine detection.

The sources said the Navy ships and aircraft did not detect any ships or airplanes tracking or stalking the Kuwaiti merchant.

In a related development, the Coast Guard said Tuesday it had received a request earlier this month from the Pentagon that it waive U.S. safety standards if necessary to speed the registry of Kuwait's oil tankers under the American flag.

The request was dated May 14 and signed by Deputy Defense Secretary William H. Taft IV, said Capt. Jim Card, a Coast Guard official involved in overseeing commercial ship inspections.

According to Card, the Pentagon is empowered by law to request waivers of Coast Guard standards ''in the interests of national defense and security.''

He said the waiver would be granted.

Kuwait, an ally of Iraq, has turned to the United States for more protection of its ships because they have become a special target of Iranian forces. The Pentagon sources said the waiver request was filed with the Coast Guard to speed registration.