Iraqi planes have succeeded in breaking through the heavy defenses of Iran's vital oil facility on Kharg Island, damaging one of the terminals there for the first time and hitting a 142,000-ton tanker berthed nearby, according to shipping and oil industry sources.

While accounts of the extent of damage varied, there was no corroboration for the assertion in a military communique broadcast by Baghdad radio that Iraqi planes had "turned Kharg into wreckage."

In a report issued in London, Lloyd's Marine Intelligence Service said that two waves of four rocket-firing planes each attacked a terminal, commonly known as "T" Jetty, on the eastern side of the island for the first time in the war. Reports from the area said the jetty was put out of commission. The same reports identified the tanker hit as the Maltese-registered Torill, which is under charter to Iran. It was reportedly hit by three French-made Exocet rockets.

A shipping analyst in London speculated that the Torill was the Iraqis' main objective, with the terminal hit more or less by accident. The Associated Press quoted one Persian Gulf observer as saying that while Kharg's loading capacity had been reduced by 40 percent, operations could be back to normal in about two weeks. No other assessment of the damage was available.

Tehran radio said one Iraqi plane had been shot down after causing "minor damage" to a tanker.

The managers of the Torill, Marine Management Shipping A/S, said in Oslo that a fire set off by the missiles had been extinguished and the vessel's 23 crew members were safe.

The strike came on the eve of Iran's presidential election and several analysts said the raid's timing may have been selected with the intention of embarrassing Iran's leadership.

There is a consensus that Iraq, which has been locked in a war with Iran for almost five years, possesses the military power to wipe out its foe's capability to export oil. Tehran depends on those exports, now estimated to be running between 1.5 million and 2 million barrels a day, to finance not only the war but its domestic programs as well. About 90 percent of Iran's oil exports are moved through Kharg Island.

Iraq is believed to have been dissuaded from making an all-out assault on Iran's terminals by other Arab states in the gulf area, which have generally supported Baghdad in the conflict. They are said to be fearful that Iran, in retaliation for the loss of its terminals, might set about destroying theirs.

Reuter quoted seamen and other sources in the gulf area as saying that the Iraqi attack on Kharg Island was, in the words of one source, a "massive blitz." The radio operator for a Panamanian-registered ship berthed at Kharg said at least 10 ships were hit and oil installations badly damaged. He told Reuter by telephone that his own ship, the Ogol, was hit in the bow and caught fire and that its crew were among the 200 seamen being evacuated from Iran.

Kharg Island's largest facility, the Sea Island terminal on the western side, was apparently not targeted in Thursday's strike. That terminal was attacked by Iraqi planes in June 1984 but it resumed partial operations earlier this year. Kharg Island is 20 miles wide and 40 miles long and lies about 140 miles southeast of Iraq.

After three years of bloody and inclusive ground fighting Iran and Iraq opened what has become known as the "Tanker War" in 1984. The two enemies hope to throttle the other's economy by choking off its oil trade. Informed sources say that so far about 90 tankers have been hit in the air attacks.

Faced with the reluctance of shipping companies to put their tankers at risk by sending them to Kharg, Iran has begun moving the crude from Kharg to a terminal on Sirri Island, farther down the gulf, where it is transferred to the companies' tankers.