Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, told a skeptical Senate Armed Services Committee yesterday that escorting Kuwaiti tankers through the Persian Gulf would not be a high-risk undertaking, although "there are no absolute guarantees that such an operation will be casualty-free."

As Crowe was testifying, Pentagon officials were studying intelligence reports about a possible Soviet naval buildup in the gulf. A Kara-class guided-missile cruiser appeared headed toward the strategic waterway, officials said.

Crowe urged Congress "not to be stampeded by overly dramatic accounts" of the gulf war. Following the Iraqi attack on the USS Stark, the Iranians "appear to be even more cautious than previously," he added.

Crowe did not elaborate on Iran's caution before the committee went into closed session to discuss U.S. contingency plans for dealing with Iran's Silkworm antiship missiles, expected to be operational by July 1, and other threats to vessels in the gulf.

After the closed hearing, which involved such high security clearances that even military aides to the senators were excluded, Chairman Sam Nunn (D-Ga.) said, "The Silkworm military threat is one of the chief concerns we have."

"We should not be declaring our intentions one way or the other," Nunn added, referring to a report in The Washington Post yesterday that the Reagan administration is debating whether to strike preemptively at the Silkworms.

"It seems to me the Iranians would certainly be increasing the threat to the region and to free transportation if they deploy those Silkworms in the area of the Strait {of Hormuz}," Nunn added.

The Silkworm missile can be transported by truck. Iran bought the weapon from China and has test fired it from the island of Qeshm into the Strait of Hormuz, which is as narrow as 30 miles. Silkworm, which carries a warhead of 1,100 pounds, has a range of 50 miles.

Although Silkworm has not been deployed along the strait, U.S. officials said that the intelligence community this week warned military leaders that Iran has received the parts it needed to make the missile operational. The imminence of the threat has touched off a debate in the administration on how to deal with the Silkworm once it goes into service, with options ranging from preemptive strikes to last-ditch defenses on U.S. ships escorting vessels through the gulf.

Without disclosing Pentagon intentions, Crowe said, "The Joint Chiefs of Staff believe that we can carry out this mission" of having U.S. Navy ships escort 11 Kuwaiti tankers through the gulf. "Of course, there are no absolute guarantees that such an operation will be casualty-free or that Iran will not escalate the sea war which will present us with further difficult choices.

"On the other hand," Crowe continued, "we have the capability to keep the oil line to Kuwait open, to assure our Arab friends of our commitment and to keep the risks low."

Flanking Crowe at the witness table were the military chiefs of the Army, Navy, Air Force and Marine Corps, or their representatives. Nunn invited them to elaborate on Crowe's remarks, noting that he wants to make sure every service chief could speak his mind. In the open session, none opted to go beyond Crowe's statement.

Crowe's portrait of the gulf as "a thriving and bustling commercial crossroads, not a no-man's land," contrasted with senatorial warnings that the Reagan administration is plunging ahead with a risky operation.

Nunn said U.S. ships could be subjected to "fanatical attacks" by Iran, "increasing the possibility" that the United States would be drawn into the Iran-Iraq war.

"We must realize that risks have increased" in the gulf and that Iran might resort to the same "suicide" attacks at sea that it has employed on land, said Sen. John W. Warner (R-Va.), who just returned from an extensive tour of the gulf region.

Reagan "has the duty to tell the people that we'll go it alone" in the gulf if military retaliation is carried out, posing "the risk of downed fliers becoming POWs {prisoners of war} in Iran," Warner added.

In a report from Cyprus, The Associated Press reported that Iranian Parliament Speaker Hashemi Rafsanjani said yesterday that his nation would attack any Arab bases and ports that were made available to the United States.

Iranian Prime Minister Hussein Masavi was quoted as saying, "We will use our forces against the superpowers if they conspire against us."