The Reagan administration is debating whether or not to strike preemptively against Iran's Silkworm missiles if they are deployed against shipping in the Strait of Hormuz, U.S. officials said yesterday.

Official sources said U.S. intelligence agencies now estimate that the antiship missiles could become operational as early as July 1.

The National Security Council has focused on the issue, sources said, with options under discussion ranging from a preemptive strike against the HY2 Silkworms to waiting until a ship flying the U.S. flag is actually threatened by the missile. The Silkworm has been test-fired from the island of Qeshm into the Strait of Hormuz but has not yet been deployed.

A lack of key parts has prevented the Iranians from putting the Chinese-made Silkworms into service, sources said. But the parts recently arrived in Iran from China, they said, making it only a matter of time before the Silkworms could target vessels passing through the strait, which is only 30 miles wide at its narrowest point.

The United States currently plans to reflag 11 Kuwaiti tankers as American ships, and these, plus their escorting U.S. warships, could be easy marks for the Silkworms, sources said. The Silkworm is credited with a range of 50 miles and a warhead with the explosive power of 1,100 pounds of TNT. The warhead of the Exocet missile that disabled the USS Stark on May 17 had the power of 350 pounds of TNT.

Debate in the intelligence community continues on how many Silkworms Iran has bought from China, sources said. The rough estimate, they said, is that at least one will be ready around July 1 and three or four more could be deployed later.

Informed sources said the behind-the-scenes debate within the administration is swirling around four main scenarios, each with advocates in the White House, Pentagon and State Department. The four:Warn Iran through diplomatic channels not to deploy the Silkworms, and hold off any U.S. military action while Tehran weighs the request. Prepare a preemptive strike by bombers or U.S. Navy guns and unleash it as soon as there is evidence that the mobile Silkworm is ready to fire and deployed to attack shipping in the Persian Gulf. Allow the Silkworms to become operational, on the theory that Iran has the right to deploy the missile as a combatant in the Iran-Iraq war, but plan to attack it at the first sign it is about to be fired at a ship flying the U.S. flag. Hold back from any preemptive action and rely on electronic jamming and other countermeasures, including putting up false targets and destroying the missile in flight, to keep the relatively small number of Silkworms from doing any damage to gulf shipping. Navy weapons specialists have been discussing countermeasures with industry experts, according to industry sources.

One question threading through the administration debate, officials said, is whether the United States would violate international law by launching a preemptive strike against the Silkworms. One interpretation, sources said, is that such a strike would be in the nature of self-defense, but another is that it would amount to the United States attacking a sovereign power at the time the Reagan administration is declaring itself a neutral in the Iran-Iraq war.

Robert B. Sims, assistant secretary of defense for public affairs, answered "absolutely" when asked at his news briefing this week whether the United States would be living up to its neutrality pledge once it starts escorting tankers operated by Kuwait, a country that has supported Iraq in the war.

"The fact is," he said, "Kuwait is not a belligerent in the war. The war is between Iran and Iraq. The Kuwaitis asked for protection for these 11 tankers. We agreed to do that. They asked that they be reflagged. That's in the process of happening. Those ships when they are being escorted will be under the U.S. flag, entitled to protection {like} any other ship that's under the U.S. flag, and they'll be paying U.S. taxes."