DUBAI, UNITED ARAB EMIRATES, AUG. 18 -- Two Iranian high-speed patrol boats attacked a Liberian-registered tanker today in the Gulf of Oman outside the Strait of Hormuz, according to shipping sources in the Persian Gulf.

Iranian commandos in the gunboats reportedly opened fire on the 20,578-ton Osco Sierra with rocket-propelled grenades and machine guns after the tanker refused to answer demands that it stop. Sources said damage to the ship was minor and there were no casualties.

It was the first attack by Iranian boats in the Gulf of Oman and represented an expansion of the kind of quick-strike naval operations Iran employed earlier this year against foreign tankers within the Persian Gulf.

At the northern end of the gulf, the Kuwaiti Defense Ministry announced that its forces had found and blown up a mine off Kuwait's main harbor, where four loaded Kuwaiti oil tankers under the U.S. flag are awaiting a U.S. Navy escort for a trip back through the Strait of Hormuz.

Meanwhile, Iranian television reported tonight that a group called Hezbollah (Party of God) in Hejaz, Saudi Arabia, claimed responsibility for Saturday's explosion at a major Saudi gas liquefication plant, undercutting Saudi claims that the blast was the result of malfunction and not sabotage.

The pro-Iranian underground group delivered a statement to news offices in west Beirut, The Associated Press reported, saying, "We shall continue the march to liberate the holy shrines from the hands of the Saudi {royal} family." Saudi Arabia has insisted that two explosions, at the Juaymah natural gas complex 10 miles north of Saudi Arabia's main oil export facility in Ras Tannurah, were accidental and caused four injuries. Other sources put the casualty toll much higher, including several deaths.

Saudi security officials have long been concerned about possible sabotage operations in the oil-rich eastern provinces, where many poor Shiites live and work at petroleum facilities. Until now, Saudi Shiites have not shown any overt sympathies toward Iranian leader Ayatollah Ruhollah Khomeini and his call to export revolution across the gulf and topple the Saudi royal family.

In an apparent sign of the mounting tensions, Saudi Arabian King Fahd has summoned home more than 30 members of the royal family vacationing in the south of France, the Manchester Guardian reported.

In the air war, Iraq reported 105 missions by its fighter-bombers against four Iranian oil installations -- the third straight day of such raids. Tehran radio said several civilians were killed and many wounded in the areas that Iraq said were hit.

Attention in the shipping war in recent weeks has focused on the use of mines in the crowded sea lanes of the Persian Gulf and in areas of the Gulf of Oman, where ships wait before entering the connecting Strait of Hormuz.

The assault ship USS Guadalcanal and its force of mine-sweeping helicopters was off the coast of Dubai this afternoon, continuing exercises apparently designed to get the force into shape for its role of clearing the way for the tanker convoy.

Pentagon officials said the Guadalcanal is carrying 18 helicopters, including eight of the sophisticated Sea Stallion mine sweepers, four Sea Cobra attack helicopters, two Sea Knight and four Huey transport helicopters.

The Sea Cobra attack craft are armed with Sidewinder air-to-air missiles, Sidearm antiradiation weapons and various guns which could be used to counter terrorist speedboat attacks, the officials said. The Pentagon already has dispatched light observation helicopters from the Army's Special Forces to assist in spotting potential small-boat assaults and the Navy has armed some of its light aircraft with additional firepower to assist in protecting the convoys and U.S. warships from such attacks, according to Pentagon sources.

The USS Raleigh, carrying small coastal mine-sweeping ships and Special Forces speedboats to repel attacks from boats, is expected to arrive in the gulf region about Aug. 30, according to the officials.

The Guadalcanal was rushed to the gulf after the first of the reflagged tankers, the Bridgeton, struck a mine on its maiden journey through the gulf to Kuwait as an American-flagged vessel. The Bridgeton, its damage temporarily repaired, is one of the vessels waiting for a return journey.

Journalists who watched the Guadalcanal move south through the Persian Gulf today from its earlier anchorage off Bahrain said that it was accompanied by two escort ships and was being shadowed by an Iranian naval vessel. As many as seven helicopters fanned out in front of the Guadalcanal carrying out simulated mine-hunting exercises.

Four ships, one Soviet, two American and one from the United Arab Emirates, have struck mines in the past several weeks, adding a new dimension to the battle of the shipping lanes that has developed as part of the seven-year-old war between Iran and Iraq.

The mines are widely believed to have been placed by Iran, utilizing the ancient dhows indistinguishable from hundreds of others that crisscross the gulf in a pattern of trade that has been part of the region for centuries. The sail- and motor-driven vessels are believed to be capable of carrying up to three mines, each weighing about half a ton.

Shipping sources say that United Arab Emirates' naval vessels have been searching dhows off its waters since two ships hit mines off the port of Fujayrah in the Gulf of Oman. Fishermen were said to have reported seeing a number of dhows unfamiliar to them in the area.

In addition to the U.S. mine-sweeping operation, Britain and France are sending mine sweepers to protect the Europeans' vessels that have been in the area for some time, escorting their flag vessels.

Britain is understood to have received a "generally favorable" response from gulf states to requests for occasional use of port facilities for its mine sweepers.

Saudi Arabia, meanwhile, said one of its diplomats injured during an attack on its embassy in Tehran had died in an Iranian hospital. The diplomat, Moussad Ghamdi, either jumped or fell from an embassy window when crowds attacked the building after the July 31 incident in Mecca in which hundreds of Iranian pilgrims were killed.

The Iranian news agency, IRNA, laid blame for the death on Saudi Arabia. "Ghamdi was taken to the airport {for a trip home at Saudi insistence}, where his condition deteriorated and he suffered a hemorrhage," IRNA said.