The missile attack on a U.S.-owned supertanker bearing a Liberian flag inside Kuwait's territorial waters yesterday dramatically exposed the limited nature of the Reagan administration's commitment to provide for the safe flow of oil in the war-torn Persian Gulf.

Administration policy now assures U.S. protection only for vessels actually under an American flag, and the escort service by U.S. warships for Kuwait's 11 tankers flying the Stars and Stripes stops at the edge of that northern gulf nation's territorial waters.

Thus, a U.S.-owned oil tanker registered in Liberia or Panama does not qualify for American military escort within the Persian Gulf. Furthermore, once a ship is inside Kuwaiti territorial waters, Kuwait, rather than the United States, has primary responsibility for its protection.

Owners of such American ships flying Liberian or Panamanian "flags of convenience" are petitioning the White House and Pentagon for a change in the rules to allow U.S. warships in the gulf to provide a "limited form of assistance" to them as well.

There are about 40 American-owned supertankers using the flags of other nations and involved in the gulf oil trade, according to Lester S. Hyman, an attorney for Majestic Shipping Co., one of the eight U.S. companies possessing such vessels. Each of them makes an average of four to five trips each year to the gulf, he said in an interview.

Secretary of State George P. Shultz reiterated at a news conference yesterday that the administration had no plans to consider a change in its present policy of restricting U.S. military protection to only U.S.-flagged vessels.

"We have very specific rules of engagement. I'm not going to discuss them in detail here, but we have no desire to change them," he said.

Regarding the larger administration objective in the gulf, Shultz said the United States wanted to make sure Iran did not succeed in becoming the dominant power there by "intimidating and bullying" the Arab states there. It also wanted to ensure the Soviet Union did not become "in a sense, protector" of vital oil supply routes to the West, he added.

The Iranian missile attack on the American-owned Sungari off the Kuwaiti oil terminal of Mina al Ahmadi yesterday appears to have been such an Iranian act of "bullying" for which the United States has no ready-made response.

Shultz sought to underline that the attack was against Kuwait and not a U.S.-flagged vessel, calling it "a very hostile action toward Kuwait."

Pentagon spokesman Fred S. Hoffman emphasized that the United States has no obligation to respond to the Iranian attack, apparently with a Chinese-made Silkworm missile, on the Sungari.

"The missile did not come near any U.S. flag vessel," Hoffman said. "Our shipping was not affected by Silkworms."

Like Shultz, he emphasized U.S. protection is limited strictly to American flagships only.

The United States has no formal commitment to protect Kuwait, either.

American shipowners are trying to change these rules. In a Sept. 30 letter to Deputy Secretary of Defense William H. Taft IV, Hyman, the attorney for Majestic, said his client was increasingly reluctant to send its six supertankers into the gulf and asked permission to have them sail "closely behind" the U.S. Navy's convoys, if not actually as part of them.

"Our company feels that if we were allowed to follow along that would give them the protection they need," he said. "We want the rules of engagement changed."

Hyman said Majestic has sent letters to the White House, Pentagon, Navy and State Department and held talks with administration officials on the question, but so far received no formal reply.

Administration and Pentagon spokesmen have been deliberately vague about what response, if any, the U.S. will make if Iran ever mounts an attack on a U.S.-flagged ship inside Kuwaiti territorial waters. They have been similarly vague about possible U.S. reactions to an Iranian land, air or sea attack on Kuwait itself.

Asked on Sept. 9 what the U.S. response would be if Iran attacked a U.S.-flagged ship inside Kuwaiti territorial waters, State Department spokesman Charles E. Redman refused to say "what we may do or may not do under particular cases or under particular circumstances."

But he noted that, "We have consistently stated our support for their {the Arab gulf states} sovereignty and territorial integrity."

Redman also said the United States was committed to protecting U.S.-flagged shipping "throughout the world, wherever it is." His comment seemed calculated to keep Iran in doubt about what the U.S. reaction would be if and when it ever attacks a U.S. flagship inside Kuwaiti territorial waters.

The Middle East Task Force providing the escort service for the Kuwait tankers under American flag has recently made a small but significant change in its proceedures.

Until recently, U.S. warships escorted the Kuwaiti tankers to a point about 40 miles from the Kuwaiti coast known as the "al Ahmadi beacon," marking the start of the channel to Kuwait's al Ahmadi port.

At that point, Kuwait took over the responsibility for protection.

Now, however, U.S. warships escort the tankers to "the outer limits" of Kuwait's 12-mile territorial waters, according to a Pentagon official.

Since the actual loading platform for supertankers is about 10 miles off al Ahmadi port, it appears U.S. warships are now escorting the Kuwaiti tankers almost all the way to their final destination.