Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, told a Senate hearing Friday that he opposes launching a preemptive strike against Silkworm missiles in Iran and doubts that Iran would fire them at ships flying the U.S. flag in the Persian Gulf, informed officials said yesterday.

Crowe, they said, also told a closed-door session of the Senate Armed Services Committee that President Reagan's plan to send U.S. warships into the gulf to escort Kuwaiti tankers is an acceptable risk.

"Crowe said he didn't think we should take a preemptive strike against the Silkworms because it might get us into the gulf war over a weapon he doubted Iran will use against us," said one source familiar with Crowe's secret testimony.

Crowe declined to disclose his views on the Silkworms or the Joint Chiefs' proposed options to combat them.

The debate over the Silkworms continues behind closed doors, officials said, with some military officers arguing that it is too risky to allow the missiles to become operational on the edge of the Strait of Hormuz. The Silkworm has a range of 50 miles; the strait is as narrow as 30 miles.

The Joint Chiefs are focusing not on the total number of Silkworms shipped, sources said, but on how many will be ready to fire early next month when the United States is expected to start escorting Kuwaiti tankers. The intelligence community has estimated that, following a recent shipment of kep parts from China, one Silkworm will be ready for shoreline duty around July 1 and three or four others probably will be deployed shortly afterward.

Military officers arguing for a preemptive strike contend that it would be too risky to count on destroying the Silkworm after it is fired. Other military and civilian officials counter that there is no hard evidence that the Iranians would be much better at firing their Silkworm missiles than the Libyans were in operating their Soviet-supplied antiaircraft missiles during the U.S. bombing strike.

The Silkworm, specialists said, can be guided by radar and heat-seeking devices or can be fired like an artillery shell without internal guidance.

Reagan's pledge to escort 11 Kuwaiti tankers flying the U.S. flag through the sometimes dangerous gulf, plus the successful attack by an Iraqi fighter on the USS Stark on May 17, has sent the Navy scurrying for additional weaponry for the strategic waterway.

Navy Secretary James H. Webb Jr. said in an interview that he will direct Navy experts this week to determine whether the LAMPS III helicopter can be armed to attack high-speed Iranian patrol boats, and perhaps even hostile helicopters or planes. The LAMPS III is equipped to find and destroy submarines, but Webb said that Navy men who fly the helicopter have told him it could be transformed into an effective weapon in the close quarters of the gulf.

The Navy, to meet the demand for more firepower in the gulf, will soon increase its force of warships from the usual six or seven to nine.