KUWAIT, JUNE 4 -- Moscow has agreed secretly to lease additional Kuwaiti oil tankers to operate under the Soviet flag if the United States reneges on its now-delayed deal to "reflag" half the Kuwait-owned tanker fleet, it was learned today.

The Soviet agreement was signed on April 1, but grew out of earlier arrangements worked out last December in a meeting with Kuwaiti officials in Moscow. It was the prime reason an initially reluctant Reagan administration accepted Kuwait's request to protect 11 tankers against Iranian attack, according to western diplomats.

The agreement forced Washington either to accept Kuwait's maneuver aimed at involving the United States directly in the Persian Gulf, or run the risk of allowing the Soviets a virtual free hand in the previously western-dominated gulf.

The previously publicized part of the Kuwaiti-Soviet agreement called for Kuwait to charter three Soviet tankers on a renewable one-year lease.

But the agreement, a copy of which was obtained by The Washington Post, also stipulates that the Kuwait Oil Tanker Co. may lease "two more" such tankers "at short notice."

The deal also enables Kuwait to lease to the Soviet Union an unspecified number of additional tankers "to be rechartered to the Kuwaiti side thereafter whenever the government of Kuwait so requests."

Implementation of such leasing arrangements "shall be agreed upon as soon as the Kuwaiti side expresses their desire," the agreement said. That allows Kuwait to turn swiftly to Moscow if the United States refuses to honor its commitment to register half of Kuwait's tanker fleet under the U.S. flag.

The Iraqi missile attack May 17 on the USS Stark, in which 37 American sailors were killed, has prompted congressional hearings on the "reflagging" operation.

Despite the superpower protection that reflagging would afford to Kuwaiti shipping, western diplomats are convinced that Kuwait's real purpose is to involve Moscow and Washington more deeply in efforts to end the nearly seven-year-old war between Iran and Iraq.

The Kuwaiti government contends that the superpowers could stop the conflict, although others have expressed doubts.

Such political considerations -- rather than damage suffered by Kuwaiti shipping -- help explain why Kuwait sought superpower protection in a major departure from its traditional neutralist foreign policy, diplomats said. Kuwait has, however, supported Iraq in the gulf war.

Since Baghdad initiated the so-called tanker war more than three years ago in a so far unsuccessful effort to curb Iranian oil exports, only seven Kuwait-owned tankers have been hit by Iran in reprisal for this small, virtually defenseless country's aid to Iraq. A total of about 230 vessels have been attacked in the gulf by both sides since 1984.

No Kuwaiti ships were among the 34 Iran attacked between Christmas Day 1985 and last Sept. 17, when the 290,085-ton tanker Al Funtas was damaged. The Al Faiha, a 263,679-ton tanker, was the last Kuwaiti ship attacked, on Oct. 22.

Moreover, oil trading sources here questioned whether the superpower mantle can provide the protection Kuwait has sought for its oil exports. The sources suggested that the tankers to be transferred to U.S. registration could handle no more than 30 percent of Kuwait's crude exports.

"The Iranians have plenty of other targets," a western diplomat remarked, noting that most of Kuwait's crude and petroleum product exports leave here either in Kuwaiti-chartered foreign tankers or ships chartered by its overseas customers.

Kuwait and other Arab gulf countries are so dependent on food imports that Iranian attacks on cargo ships could have a devastating impact on their economies. Following an Iranian attack on the Corriedale Express on April 15, for example, a few thousand sheep on board were lost and the price of mutton rose so sharply that the government here intervened.

If the U.S.-flagged tankers handle as much as 30 percent of Kuwait's crude exports, the sources said they would be assigned the shuttle run between Kuwait and Khawr Fakkan, outside the Strait of Hormuz in the Gulf of Oman and thus beyond Iranian attack.

Kuwait already is shuttling crude to large storage vessels at Khawr Fakkan to avoid exposing tankers to attack inside the gulf. At Khawr Fakkan the crude is pumped aboard other tankers for delivery to overseas customers.

Iran operates a similar oil shuttle between the Kharg Island loading facility in the northern gulf and Larak Island in the Strait of Hormuz.