Senate Foreign Relations Committee Chairman Claiborne Pell (D-R.I.) introduced legislation yesterday to block the Reagan administration from putting Kuwaiti tankers in the Persian Gulf under the protection of the U.S. flag.

The bill, cosponsored by Senate Majority Whip Alan Cranston (D-Calif.) and Sen. Mark O. Hatfield (R-Ore.), also urges President Reagan to seek establishment of a U.N. peacekeeping force to protect nonbelligerent shipping in the war-torn gulf.

The legislation, reflecting mounting concern in Congress over risks of the administration's reflagging plan, goes far beyond measures previously approved by the House and Senate to require the administration to inform Congress fully of its plans to protect U.S. interests in the Persian Gulf.

The Foreign Relations Committee is not expected to act on Pell's proposal until after the administration report is received, theoretically by the middle of next week, sources said.

Pell's bill would bar the United States from making or implementing agreements with Persian Gulf countries to reregister their vessels under U.S. flag protection and denies funding to carry out any such pact.

The administration's plan to reflag Kuwaiti tankers and provide them with U.S. naval escorts is "poorly conceived and dangerous . . . undertaken without balancing the possible benefits against the very real risks of greater American involvement in the Persian Gulf hostilities," Pell said as he introduced the legislation.

He warned that Iran could interpret the reflagging as a provocation and attack U.S. escorts, leading to American retaliation. "Are we prepared to match Iran in an escalating spiral of violence?" he asked. "Are we prepared to commit young American lives to the defense of Persian Gulf Arab nations?"

In addition to calling for a U.N. peacekeeping force and U.N. efforts to bring an early end to the Iran-Iraq war, Pell also urged the administration to seek a mandatory arms embargo against Iran from the U.N. Security Council.

Meanwhile, Senate Armed Services Committee members John Glenn (D-Ohio) and John W. Warner (R-Va.), returning from a week-long fact-finding mission to five Persian Gulf countries, agreed the U.S. runs the risk of war in the region. But Glenn, Warner and Sen. Jim Sasser (D-Tenn.), who made an earlier trip, disagreed over whether the reflagging plan might be blocked or abandoned in light of opposition to it.

Warner said the administration is sticking to its plan and intends to start implementing it in two to three weeks. Glenn said there is a "chance" Congress might move to block the reflagging. He expressed hope for an alternative course of action such as U.N. sanctions but cautioned that Arab countries see reflagging as a "litmus test" of U.S. resolve in the region. Sasser said he believes the administration itself may be having second thoughts about what he likened to a "Hertz rent-a-navy" approach to the conflict.

Warner cautioned that the United States would have to "go {it} alone" in retaliating for attacks by Iran, citing indications from Persian Gulf nations that they would not join in any response outside of their own territories or waters. They are afraid the United States will "stir up a hornets' nest and then leave," Glenn said, adding that the United States would have to "go in with enough force to prevail."