Presidential national security adviser Colin L. Powell said in a letter released yesterday that the Soviet Union might halt agreed-upon reductions of its strategic nuclear arms or even increase its long-range forces if it is not satisfied that the United States is complying with the 1972 Antiballistic Missile (ABM) Treaty.

Sen. Carl Levin (D-Mich.), who received Powell's letter and made it public, called it "a needed clarification" of what happened at the Washington summit. It appears to refute President Reagan's remark shortly after the meeting that the summit had "resolved" the U.S.-Soviet argument over the conflicting intepretations of the ABM Treaty as they relate to his Strategic Defense Initiative.

Levin, a leading critic of the ABM reinterpretation in the Armed Services Committee, said the letter shows "there is a long way to go" before strategic defense issues with the Soviets are resolved, and that "if we don't reach a START {strategic arms reduction} agreement, the likely cause will be the unresolved conflict over ABM and SDI."

Levin added in a telephone interview that "it's important to understand that if the {START} negoiations fail, this not be laid at the doorstep of some kind of backing away by the Soviets from an agreement reached during the Washington summit. . . . There was no agreement that we can proceed with SDI without precipitating their right to build up their strategic forces."

The key move on strategic defense issues at the summit, made public Dec. 10 in the final joint statement, was U.S. and Soviet agreement to instruct their delegations at the Geneva negotiations "to work out an agreement that would commit the sides to observe the ABM Treaty, as signed in 1972, while conducting their research, development and testing as required, which are permitted by the ABM Treaty, and not to withdraw from the ABM Treaty, for a specified period of time."

Several U.S. officials said immediately after the summit that the two leaders had "agreed to disagree" about SDI and the restrictions on it that are imposed by the ABM Treaty. At the same time, they said it was significant that Soviet leader Mikhail Gorbachev accepted a statement that research, development and testing could continue "as required."

The officials also said it was significant that Gorbachev and his delegation did not make a major issue of the question of how the ABM Treaty is to be interpreted, but instead approved a final joint statement that was ambiguous on the subject.

Powell, interviewed on NBC's "Meet the Press" on Dec. 13, said that "the president has preserved his right to go ahead with the SDI program." He also said, "The Soviet position is clear. They think SDI is the wrong way for us to be going. . . . They would prefer that all of our testing and research and development were restricted to the so-called narrow interpretation of the ABM Treaty. The president did not accept that, and the joint statement does not put a girdle around us in that way."

Powell's letter to Levin said the Soviets insist on retaining the option to "suspend implementation" of negotiated strategic arms reductions, "and perhaps even {to} begin increasing their strategic offensive forces," if the United States were to violate the Soviet Union's view of the restrictions imposed by the ABM Treaty.

Powell said he thinks Gorbachev "recognized" that the joint statement as written and approved "can be read to reflect the U.S. position" on strategic defense issues.

"However, it is clear that the United States and the Soviet Union do not, as yet, share a common understanding about these {ABM Treaty} obligations along the lines of the U.S. position," Powell wrote. "We will continue to work towards such an understanding over the coming months."