DUBAI, UNITED ARAB EMIRATES, AUG. 19 -- A third convoy of reflagged Kuwaiti tankers moved into the Persian Gulf today under U.S. Navy escort and joined up with the mine-hunting USS Guadalcanal for the 600-mile trip to Kuwait.

The three tankers, including one of the world's largest ships built to carry refined petroleum products, were escorted by three Navy vessels in addition to the Guadalcanal and its fleet of helicopter mine hunters.

The three ships bring to seven the number of vessels currently in the gulf that have been reflagged under U.S. colors as part of an American policy of using naval power to ensure freedom of navigation in the embattled waterway. The other four vessels are waiting in harbor in Kuwait for a return trip through the gulf.

{Pentagon officials said Wednesday that they expect the Navy to begin escorting the four tankers, including the mine-damaged Bridgeton, out of the gulf shortly after the current convoy of three tankers completes its journey, Washington Post staff writer Molly Moore reported in Washington.

{Iranian ships shadowed the convoy during part of the trip, but naval officials in the Persian Gulf reported no unusual Iranian air activity, according to Pentagon officials. They said no activity was detected at any of Iran's known Silkworm missile sites along the Strait of Hormuz.}

The Bridgeton struck a mine July 24 on its trip northward through the gulf, underscoring the dangers faced by the tankers and their escort vessels. After that incident, the Guadalcanal was brought in with its Sikorsky RH53D Sea Stallion helicopters to sweep the sea lanes ahead of the tanker fleet and its escorts.

{U.S. fighter and surveillance planes based on the carrier USS Constellation in the northern Arabian Sea provided air cover during the transit through the strait, Pentagon officials said.}

The Soviet Union already has mine sweepers in the gulf to assist vessels sailing under its flag, and Britain and France have mine-sweeping ships en route to the gulf to help their naval escort vessels that have been in the gulf region for some time.

The mines are widely believed to be laid by Iran as part of the "tanker war" that has developed as one theater of the Iran-Iraq conflict, now close to beginning its eighth year. Seven vessels have been struck by mines since May 16. Mines have been found in the Gulf of Oman off the U.A.E. port of Fujayrah as well as at several points in the narrow shipping channels of the Persian Gulf.

In addition to using mines, Iran has employed naval vessels -- often small speedboats equipped with machine guns or rocket propelled grenades -- o harass and attack shipping in the gulf.

Most damage to shipping, however, has been done by Iraq, which has used its air power against ships carrying out Iranian crude oil or bringing in refined petroleum products for Iran.

The three ships to enter the gulf today, the Gas Princess, Gas Queen and the 290,000-ton Townsend, passed through the Strait of Hormuz at dawn and joined up with the Guadalcanal once inside the Persian Gulf.

At one point, the convoy reportedly stopped as a helicopter gunship fired at an unidentified target in the water ahead of the vessels. At another point in the journey, a Navy helicopter gunship approached a helicopter carrying journalists, warning it away from the convoy. By midday the convoy was off the U.A.E. port of Ras al Khaymah.

Soviet ships as well as the Iranian vessels were reported by the Pentagon to be near the convoy's path.

A statement by Rear Adm. Harold Berensen, commander of the U.S. Middle East Force in the gulf, said the convoy had passed "in the vicinity of Iranian ships" in the Gulf of Oman last night. He also said there had been no unusual Iranian air activity.

The Navy escort ships for the convoy are the guided-missile destroyer Kidd and the guided-missile frigates Crommelin and Klakring.

As the new convoy moved into the gulf, the captain of the Norwegian-operated tanker Osco Sierra, which was attacked by Iranian patrol boats last night as it entered the Strait of Hormuz, was reported to have said by radio that his ship had been hit by a rocket fired from a small Iranian boat. He reportedly said there had been no injuries and little damage.

{In Washington, State Department spokeswoman Phyllis Oakley said Wednesday that the attack occurred in the Strait of Hormuz rather than the Gulf of Oman. "The early information was that it happened in the Gulf of Oman. Later corrections giving the coordinates placed it in international waters in the Strait of Hormuz," she said.}

Shipping sources in Dubai said the Osco Sierra is expected to come to the large dry dock facilities here for a closer check of damage.

The Iranian news agency IRNA today strongly denied that its patrol boats were involved in the attack.

In the northern gulf, IRNA said, Iranian coastal artillery had sunk four Iraqi gunboats and set another afire in the channel separating Kuwait from the Faw Peninsula.

The Iranians also said their artillery was hitting towns in southern Iraq in response to Iraqi artillery attacks on Iranian civilian centers.

Iraq denied that its vessels had been sunk by Iranian artillery fire.