The United States detected preparations to make the Iranian Silkworm antiship missile operational over the weekend and launched Navy planes from an aircraft carrier south of the Persian Gulf with the idea of knocking out the missiles if they went into action, Pentagon officials said last night.

Officials said that launching the planes was a precautionary measure and did not represent a decision by President Reagan to take out the Silkworm missiles in a preemptive strike. The Joint Chiefs of Staff and administration officials have been debating the wisdom of making such a preemptive strike since the intelligence community warned last month that the Silkworms would be ready to fire in early July.

A White House spokesman said he had no information on the event.

Planes from the USS Constellation were launched, officials said, but returned to the carrier without dropping bombs or firing missiles. The carrier is steaming outside the gulf in the Arabian Sea. It was the first time naval air power has been mobilized over the gulf since Reagan announced his intention to escort 11 Kuwaiti tankers flying the U.S. flag.

The Iranians suspended the preparatory work on the Silkworms before the administration had to decide whether to make a preemptive strike, officials said. It was not clear last night whether the Iranians stopped working on the mobile Silkworm missiles, which they have test-fired into the gulf at the Strait of Hormuz, in response to warplanes overhead or whether they had no intention of making missiles ready to fire.

The Silkworm has a range of about 60 miles and carries a 1,000-pound warhead that could be lethal to a tanker. Navy sources said that surface ships now in the gulf could knock out the Silkworms with gunfire if they became operational and expressed surprise that aircraft from the Constellation were sent aloft over the weekend.

Pentagon sources would not reveal how the activity at the missile site was detected, but it is known that U.S. satellites keep track of the Silkworm locations in Iran. Such overhead surveillance would have transmitted instantaneous pictures of technicians working on the missile. It is also possible that electronic eavesdroppers that the United States has aboard ships and planes in the Persian Gulf detected preparations to get the missile ready to fire.

While the United States added aircraft to the show of force provided by a fleet of warships already in the gulf, the Soviet Union over the weekend continued to limit itself to a few minesweepers for escorting Kuwaiti ships they have leased.

"While we're putting together an armada out there and scrambling airplanes," said one U.S. official, "the Russians seem to be relying on their flag to provide protection in the gulf. This weekend provided quite a contrast."

The decision to employ air cover in the gulf in response to the upgrading of the Silkworm threat was made by Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, U.S. officials said. Crowe is known to be against making a preemptive strike on the Silkworm missiles, but his actions over the weekend indicate that he will mobilize forces quickly if the missile is made ready to fire.