President Reagan yesterday confirmed that the Soviet Union has presented a new offer at the Geneva arms talks and said the United States is studying the proposal.

Reagan, questioned about reports of the proposal, gave no details beyond confirming the presentation. Administration officials said earlier in the day the proposal involves long-range missiles now limited by the SALT II treaty, among other weapons.

Officials said the proposal was presented by chief Soviet negotiator Viktor Karpov at a full session of U.S. and Soviet negotiators. Two weeks ago, the Soviets informally talked of agreeing to reductions in long-range systems if the United States would agree to continue adhering to the 1972 Antiballistic Missile Treaty for 15 to 20 years.

The proposal came as a key U.S. arms adviser testified yesterday before a congressional committee on U.S. compliance with the limits of the unratfied SALT agreement.

The United States could remain in compliance with SALT II, even if later this year Reagan carries through on his plan to exceed one of the accord's limits by equipping more than 130 B52 bombers to carry cruise missiles, according to Paul H. Nitze, the president's special adviser on arms control.

Nitze told the arms control panel of the House Armed Services committee yesterday that the Navy this fall probably will decide to dismantle at least one of two additional missile-firing Poseidon submarines that are scheduled for overhaul later this year or in early 1987.

By modifying the 131st B52 bomber to allow it to carry cruise missiles later this year, the United States would push through the SALT II sublimit on multiwarhead nuclear systems. However, dismantling the 16 multiwarhead missiles on a Poseidon submarine could drop the U.S. total of such systems back below that limit. Since only two bombers are being converted each month, it would take at least eight months to reach the treaty limit again.

Although Nitze said a decision on the submarines will not be made until September or October, he added, "I anticipate one of the subs will be dismantled."

Rep. Beverly B. Byron (D-Md.), who chaired the arms control panel, said she interpreted Nitze's statement on the Poseidon dismantling as meaning "this would let us deploy 16 more [B52 bombers equipped to carry cruise missiles] and extend the life of SALT II until late summer" of 1987.

Nitze's remarks came against a background of increasing congressional opposition to the president's May 27 statement that he plans to disregard the SALT II limits.

Rep. Jim A. Courter (R-N.J.), a strong supporter of the Reagan decision to disregard SALT, told Nitze that "regrettably, there is not a great deal of support for the president's decision." He cited opposition from Congress, U.S. allies and newspaper editorials.

In the Senate, Sens. Joseph R. Biden Jr. (D-Del.) and William S. Cohen (R-Maine), yesterday said Reagan's decision to disregard SALT II was a grave mistake.

Cohen, a member of the Senate Armed Services Committee, argued that the United States would violate sublimits in the treaty "that work to both U.S. and Soviet interests" as a response to Soviet violations in less important areas. SALT II was signed in 1979 but never ratified by the Senate; the administration contends that the Soviets have provoked Reagan's action by violating two provisions in the accord.

Biden declared that Reagan's arms control policies were being set by "right-wing advisers" who "are seeking to use militarily insignificant treaty violations as an excuse to destroy the entire strategic framework with which they never agreed in the first place."

Biden and Cohen have introduced a bill that would keep spending on strategic nuclear systems within the SALT II limits. They said they would delay their measure until after the Senate deals with a nonbinding resolution calling upon the president to observe the treaty.

In a related move, a bipartisan group of four pro-SALT senators met yesterday with national security adviser John M. Poindexter to discuss their next legislative move. After the session, Sen. John H. Chafee (R-R.I.), said he didn't think Reagan's SALT II decision was final.