Iran and Iraq are stepping up their tanker war in the Persian Gulf, doubling the number of ships hit, varying tactics and weaponry, and expanding combat zones.

But the naval war, now in its third year, has failed to deprive the combatants of enough oil revenue to force an end to their larger conflict on the ground, which erupted in September 1980.

That was Iraq's original objective when France provided Super Etendard aircraft designed to strangle Iran's oil exports by either knocking out the giant Kharg Island oil terminal in the northern Gulf or sinking tankers loading there.

In the glutted international oil market, with No. 1 exporter Saudi Arabia still unsuccessfully trying to impose its sales limitation scheme on other producers, the outside world has absorbed the escalation in violence complacently.

So far this year, almost as many unarmed merchant ships have been hit as the 46 damaged throughout 1985 -- with a record 15 vessels attacked in March alone. But no major, fully laden tanker has been sunk yet in the hostilities.

More worrisome to Arab Gulf states and foreign diplomats is that Iran in April began for the first time to strike shipping in the lower Gulf, close to the strategic, 32-mile-wide Strait of Hormuz that leads to the Gulf of Oman and open water.

The United States and other western naval powers, such as Britain and France, are on record as determined to use force to keep the straits open to international shipping. The U.S. Navy maintains a five-vessel force in the Gulf with headquarters in Bahrain.

After more than a month of relative calm, which the oil industry speculated was due to U.S. backing for Saudi complaints to Iran, a new Iranian helicopter attack was reported over the weekend. Shipping sources said that an American-designed Iranian helicopter used a wire-guided French AS12 missile in a daylight attack against the Greek tanker Koriana, only 10 miles off the coast of the United Arab Emirates and 25 miles from the strait.

Meanwhile, shipping sources reported that Iraq for the first time has copied Tehran's use of helicopters, attacking tankers running the gauntlet between the still functioning Kharg terminal and Sirri Island, out of hostile aircraft range 350 miles to the south.

The sources said long-range, land-based helicopters of apparent French construction were used last week in the attack south of Kharg against the 343,000-ton Liberian-flagged Medusa.

The tanker belonged to the much battered fleet chartered by Iran over the past 18 months to shuttle oil from Kharg to Sirri, where commercial customers take on crude from supertankers acting as floating storage tanks.

Desperately short of serviceable fixed wing combat aircraft, Iran pioneered the use of helicopters in the naval war early this year to provide a strike force as effective as previously employed fighter-bombers.

At first, helicopters were stationed on two disused offshore oil rigs called Rostam and Sassan located conveniently off Qatar and close to the main routes for tankers heading north for Kuwait and the Saudi oil terminal at Ras Tannura.

The Iranian raid against the Koriana was launched from Abu Musa, one of three once Arab islands near the straits seized by the late shah Mohammad Reza Pahlavi of Iran in 1971 when Britain ended its more than 150-year military presence in the Gulf.

The reason behind Iraq's apparent use of helicopters against Iranian-chartered tankers remained unclear. Shipping sources speculated that helicopters could surprise their tanker prey and escape detection by flying under Iran's radar based on the mainland at Bushehr. They could also mask their presence behind Kharg Island's silhouette.

The shipping sources also speculated that cheaper, less sophisticated weaponry could be used more effectively from helicopters against tankers than the French-manufactured Exocet "standoff" air-to-ship missiles fired from afar.

These heat-seeking missiles cost $600,000 each, according to shipping sources, and they tend to cause only minimal damage to the oil cargo by often lodging either in tankers' engine rooms or crew quarters.

Moreover, ever since Iraq started the tanker war in the spring of 1984 with Super Etendards -- now replaced with longer ranged Mirage F1Cs -- its pilots, except on rare occasions, have not been known for low level attacks either on Kharg or on Iranian-chartered shipping.

With no end of the tanker war in sight, shipping sources are concerned that the Iranians might start attacking shipping at night. Commercial shipping has been steaming through the straits in unofficial convoys at night, hoping to make Dubai by dawn, to avoid the Iranians.

The second night they make for Ras Tannura or Kuwait on the theory that Iran has not attacked in that northern stretch of the Gulf since Saudi Arabian jets shot down one, and possibly two, U.S.-built Iranian F4 Phantoms in May 1984.

So far, the small states of Qatar and the United Arab Emirates in the lower Gulf have shown no readiness to commit their air forces to ward off Iranian marauders, apparently in fear of angering Tehran. Nor have they and other Arab Gulf states deployed their naval forces.

Except for the predominantly Greek and Turkish owners of the Kharg-to-Sirri tankers, the only moneymaker in the tanker war appears to be Dubai's giant drydock. Decried as a white elephant in the post-petrodollar era, the drydock said it turned its first profit last year -- repairing damaged tankers.