The war between Iran and Iraq, now entering its sixth year, has taken an ominous new turn that U.S. government analysts say poses the most serious threat to U.S. interests in the Persian Gulf since at least the attacks on oil tankers of noncombatants in the spring of 1984 and possibly since the start of the war.

Iraq has declared its intention to destroy Iran's main offshore oil terminal on Kharg Island to halt its exports and yesterday launched the 13th raid since mid-August to demonstrate Baghdad's seriousness. Iran, in turn, has threatened to halt all Arab oil exports from the gulf if Iraq succeeds.

U.S. analysts here believe the Iraqi air attacks, despite extensive damage to the Kharg loading facilities, have so far failed to have a major impact on the level of Iranian oil exports. But oil industry sources based in the gulf increasingly dispute this contention and say Iran's exports are considerably reduced, at least temporarily.

In any case, the analysts here predict that Iran will retaliate if and when its exports are cut "a significant amount," as one put it.

"We have reached a more dangerous phase for oil shipping from the gulf and for U.S. interests in the area," said one government analyst. "There is a greater potential for an inadvertent confrontation involving U.S. interests than ever before."

Among the scenarios envisioned by U.S. sources for such a confrontation are an Iranian air attack against one of the few remaining U.S. oil tankers sailing to the gulf, an Iranian stop-and-search operation on U.S. merchant vessels entering the gulf, an Iranian attempt to close the Strait of Hormuz leading into the gulf, or an Iranian air raid on Saudi facilities in which U.S.-piloted airborne warning and control system (AWACS) planes supporting Saudi warplanes might become involved.

The United States has already warned U.S. vessels to exercise "extreme caution" upon entering gulf waters because of the latest spiral in the war. For some time, the U.S. Navy has offered to escort U.S.-flag ships in the lower gulf -- although the Navy offer does not extend to ships heading to Kuwait and other points north.

These escorts are provided either by the five-ship U.S. Mideast Force operating out of Bahrain inside the gulf, or other ships stationed in the Oman Gulf and Indian Ocean.

The sober Reagan administration assessment of the new risks comes as Iraq steps up its air raids on Kharg Island and as Iran increases its practice of occasionally stopping merchant vessels entering the gulf to search for Iraq-destined arms. Iran has also recently launched its first large assault in six months into northern Iraq.

All these signs, U.S. analysts say, point to a higher level of risk-taking in the war by both Iraq and Iran, as well as the unrelenting pursuit by the Iranian religious leader, Ayatollah Ruhollah Khomeini, of his goal of overthrowing Iraqi President Saddam Hussein and his Baathist regime.

In a reference to former president Jimmy Carter and the late Shah Mohammad Reza Pahlavi of Iran, Richard Helms remarked, "Khomeini thinks he got the shah and Jimmy Carter and he also thinks he can get Saddam Hussein. The perpetuation of this war is kind of a personal vendetta." Helms was U.S. ambassador to Iran from 1973 to 1976 and CIA director during the Nixon administration.

Helms said he was convinced that Iranian leaders were "dead serious" in their threats to retaliate; he said Iranian artillery ashore might begin shelling tankers and other merchant vessels passing through the narrow Hormuz Strait to scare shipping from the gulf.

The war, which Iraq says began with Iranian attacks on Sept. 4, 1980, and Iran says began on Sept. 22 with an Iraqi invasion of Iran, is already probably the most costly conflict in human lives since World War II.

Last January, the Pentagon estimated Iranian casualties -- dead and wounded -- at 550,000 and Iraq's at 250,000. This week, one U.S. government analyst put the total number of deaths at 300,000 to 350,000, with Iran accounting for 250,000 of them.

Assistant Secretary of State Richard W. Murphy, in testimony before Congress Sept. 18, said the war had also witnessed "the largest-scale use of chemical warfare since World War I." The United States has repeatedly criticized Iraq for using chemical weapons, a charge denied by Iraq.

Most U.S. analysts agree that Iraqi attacks on Kharg represent not only a significant change in Saddam's war tactics but also a new threshold of danger for the entire gulf region. The Arab oil producers, particularly Kuwait and Saudi Arabia, are bracing themselves for Iranian retaliation on their oil facilities, either through air attacks or terrorist sabotage.

"If Iran's exports are down several hundred thousand barrels a day for a period of several months, than I think you'll see implementation of some of their threats," said one U.S. analyst.

Kharg is about 25 miles off the southwest coast of Iran. It once was designed to load 6.5 million barrels a day through as many as 16 berths for tankers and supertankers and a huge tank farm capable of storing up to 18 million barrels of crude oil. Between 80 and 90 percent of all Iranian exports originate from Kharg, but a good portion of this is now shuttled in small tankers to Sirri Island further south beyond the range of Iraqi warplanes.

Only two of the 13 Iraqi attacks have inflicted substantial damage on the vast complex of loading berths, according to U.S. analysts who also say there is evidence that Iran's U.S.-made Hawk antiaircraft missiles defending the island have not performed well. Oil industry sources in Kuwait contend the Iraqi attacks have been successful enough to cut exports from Kharg down to 750,000 barrels a day -- a drop of almost two-thirds from recent levels.

The Kuwaiti sources said that all loading berths on what is known as "Sea Island" on the exposed western side of Kharg are now out of action.

But these oil sources, quoted by Reuter in a dispatch Wednesday from Kuwait City, said at least two berths for 200,000-ton supertankers were still functioning.