An American-owned oil tanker was attacked by an Iranian gunboat in the Persian Gulf yesterday but was not entitled to protection from U.S. warships there because it was flying a Liberian flag, U.S. officials said.

The gunboat intercepted the 273,000-ton Peconic, owned by Universe Tank Ships Inc. of New York, in the northern gulf off Kuwait early yesterday morning, fired 18 grenades and set the vessel on fire, shipping sources in London and the gulf said.

No injuries to the crew were reported, and the fire was extinguished without major damage to the vessel, which was on charter to Texaco Inc. It was not known whether any Americans were aboard.

White House spokesman Marlin Fitzwater said, "We deplore and regret the attack." But he said the Reagan administration had "no intention of retaliating."

State Department spokesman Charles E. Redman, apparently seeking to play down the attack, said it seemed to be "yet another in a long series of attacks perpetrated against nonbelligerent and neutral shipping by the Iranians."

He said U.S. warships had not intervened because the Peconic was sailing under a Liberian flag. "U.S. naval forces in the gulf were not responsible for its protection," he said.

Fitzwater and Redman said the attack underscored "the urgent need" for a comprehensive cease-fire in the Iran-Iraq war "covering all areas." This was an apparent reference to both the land war and the so-called "tanker war" in the gulf, in which nearly 330 vessels have come under Iraqi and Iranian attack in the past three years.

The latest Iranian attack highlights what some congressional critics regard as a major inconsistency in the administration's plan to provide military protection in the gulf for 11 Kuwaiti tankers that are being re-registered under U.S. flags: While those ships will be protected by escorting U.S. warships, American-owned ships plying the gulf under foreign flags will not be given the same treatment.

The first convoy of reflagged Kuwaiti tankers escorted by American warships through the gulf is scheduled to take place within the next two weeks.

Pentagon spokesman Maj. Larry Icenogle said U.S. protection policy toward U.S.-owned ships flying foreign flags operates on "a case-by-case basis," adding: "They would have to ask."

He said U.S. warships had protected such vessels in February and May when a Kuwaiti freighter transporting U.S. tanks sold to Bahrain in the gulf.

It was not immediately known whether Iran was aware the Peconic was American-owned and thus had singled out the tanker, en route to pick up oil in the "neutral zone" between Kuwait and Saudi Arabia.

But 300,000 barrels of the zone's daily production is sold by Kuwait and Saudi Arabia on behalf of Iraq with the proceeds used to help finance its war effort. The Iranians may have chosen the Peconic because of this.

The Iranian attack appeared to be in retaliation for a spate of Iraqi attacks on Iranian ships since June 20. There had been a 34-day lull in the "tanker war," dating from May 17 when an Iraqi jet hit the frigate USS Stark, killing 37 American sailors. The attack came only a few hours after Iraqi jets raided Iran's Kharg Island oil terminal.

The Peconic's captain, Michael Monogios, contacted by CBS News, said the Iranian gunboat had attacked without warning and without attempting to determine first the name, nationality or course of the ship. He said his crew counted 18 grenades thrown at the vessel.

After the fire was put out, the tanker was being escorted by a salvage tug to Bahrain, the captain reported.