A Saudi Arabian supertanker was set afire in an air strike off the Saudi coast in the Persian Gulf today, and U.S. officials said the attack almost certainly was carried out by Iranian warplanes.

The 212,000-ton Yanbu Pride, which had loaded a cargo of oil at a Kuwaiti port, was hit about 50 miles off the Saudi port of Jubail as the war against gulf shipping--an outgrowth of the 3 1/2-year-old war between Iran and Iraq--escalated.

Five oil tankers have been attacked in the gulf this week, two by Iraq and three apparently by Iran. The development has significantly broadened the hostilities and raised concerns among the world oil and shipping industries about continued operations in the Persian Gulf, where a large portion of the noncommunist world's oil is produced.

In Washington, the State Department expressed concern that the attacks will draw other nations into the war.

"The United States abhors this continued series of attacks on international shipping," State Department spokesman John Hughes said. "It reiterates its firm belief that attacks on international shipping in the Persian Gulf make a dangerous escalation of the Iran-Iraq war and a growing threat to freedom of navigation in international waters."

Major maritime insurers announced that higher "war-risk" premiums have been imposed for the northern third of the Persian Gulf as a result of the spreading attacks on oil tankers, and the Mobil Oil Corp. announced that it has ceased sending its tankers into that portion of the gulf. Details on Page A25.

No casualties were reported in the attack today on the Yanbu Pride. After the crew and other vessels extinguished the four-hour fire, set off by the rockets, the tanker limped toward Bahrain for repairs.

U.S. officials said American AWACS (Airborne Warning and Control System) aircraft monitored the attack on radar screens and detected only Iranian warplanes in the vicinity, Washington Post staff writer Fred Hiatt reported from Washington.

The U.S. planes alerted Saudi Arabia, which sent two U.S.-made F15 fighter jets to the tanker but apparently were too late to respond.

Two Iranian F4 fighters, part of the U.S.-supplied Air Force built up during the reign of the late shah, had approached the tanker and circled it before attacking, apparently making a positive identification, U.S. officials said.

The AWACS, which cannot pick up ship traffic nor see weapons actually being fired, alerted the Saudi Air Force as the F4s approached. The Saudi F15s--part of the 62 bought from the United States--flew out to the tanker and circled it, but did not give chase to the F4s, which they can outperform, U.S. officials said. They said they did not know whether the Saudis deliberately avoided a confrontation with the Iranian jets or simply arrived too late.

The Saudi-led, six-nation Gulf Cooperation Council scheduled a foreign ministers' meeting Thursday to discuss emergency measures to protect shipping in the gulf. The Kuwaiti Cabinet, after a special meeting, openly blamed Iran for the first time for attacks on two Kuwaiti tankers earlier this week.

The Yanbu Pride was only partially loaded after picking up 128,000 tons of Kuwaiti crude oil at Mina Ahmadi port in Kuwait. It was scheduled to pick up some more bunker fuel at Ras Tanura, about 60 miles south of where it was attacked. The tanker is owned by Arab International Maritime Co. in Jiddah, with Mobil Oil Corp. a minority owner.

If its planes carried out the attack, Iran, according to Middle East analysts, appeared to be sending a clear signal to Iraq and its allies that Baghdad's efforts to choke off Iranian oil exports through air power would not go unchallenged.

Hojatoleslam Ali Akbar Hashemi Rafsanjani, speaker of the Iranian parliament, was quoted by Tehran radio as warning that "if the Kharg Island route is not safeguarded then no other routes in the Persian Gulf will be secure."

Shipping near Kharg Island, through which passes 90 percent of Iran's estimated 1.7 million barrels of oil exports daily, has come under heightened Iraqi air attack since early this month.

As Iraqi President Saddam Hussein has become more desperate in his efforts to blunt Iran's manpower advantage in the war, he has resorted to his acknowledged air superiority to bomb ships trading with Tehran.

By attacking Kuwaiti and Saudi tankers, especially near the Saudi coast, Iran could be warning Iraq's two biggest financial backers to use their influence to halt Baghdad's escalated air attacks on shipping.

But the Iranian air operations do not represent the closing of the Strait of Hormuz--through which a sixth of the noncommunist world's oil passes. President Reagan, and before him president Jimmy Carter, have warned that any such Iranian action would prompt American reaction to keep that vital waterway open and operating.

U.S. military officials were said to be watching the situation with concern, but no immediate action seemed likely. The USS Lasalle, the flagship of the Navy's five-ship Middle East force, was reported headed from Bahrain to Karachi for a port call, although it was thought to still be in the gulf.

Four other U.S. combat ships remain in the gulf, and the carrier USS Kitty Hawk with its battle group and F14 fighter jets is on station nearby in the northern Arabian Sea.

The United States has four AWACS, three KC135 tanker planes to refuel the AWACS in midair and more than 500 personnel based in eastern Saudi Arabia. The AWACS' primary mission is to provide early warning of attacks on the vulnerable oil installations along the western side of the gulf, but they have also been monitoring air traffic at the northern end of the gulf where the shooting between Iraq and Iran has been heaviest.

The Saudis receive information by radio from the AWACS and then control their own F15s, U.S. officials in Washington said. Saudi Arabia is reluctant to be seen as cooperating closely with the United States on military matters and would not cede control of its jets to the AWACS operators.

U.S. officials also said today that Iran was flying cargo planes to an island in the Strait of Hormuz, where Iran is known to have artillery, and was unloading equipment there, Hiatt reported. The officials said they did not yet know the nature of the equipment or the significance of the sighting.

Despite Kuwait's condemnation of Iranian attacks, Saudi Arabia, the most important Arab power in the gulf, today adopted a neutral stance. A statement issued by the official Saudi press agency did not mention Iran but merely expressed Saudi Arabia's determination to "end such excesses, in cooperation and consultation with its sisters in the Gulf Cooperation Council."

Since the outbreak of hostilities, the Arab gulf states have been torn between fears of the Iranian revolution spilling over and destroying their governments and the growing danger and cost of supporting an increasingly desperate Iraq.

Ironically, some of the shipping attacked by Iraqi aircraft in Iranian waters belonged to either nationals or government-owned companies of Arab states in the gulf. Thus Iraq's allies were being punished by Iraq for helping finance Iran's war effort through oil exports.

As recently as May 9, Saudi Oil Minister Ahmed Zaki Yamani sought to gloss over Iraq's attacks on two Saudi tankers near Kharg Island, saying that the attacks were not deliberate and "it is difficult for military aircraft to distinguish and identify such target."