Washington — An across-the-board breakthrough in nuclear arms reductions is possible by the end of 1983 if the superpower leaders exercise the necessary political will, according to an international commission.
At the end of two years of study, the 17-member commission also warns, however, that dialogue and moderation in international relations appear to be breaking down and that tensions are accelerating.
In a 181-page report, the commission argues that it would be impossible to win a nuclear war and that talk of fighting a ''limited nuclear war'' is dangerous. It recommends, among other things, a nuclear-free zone in Central Europe and the removal of all chemical weapons from Europe.
The Independent Commission on Disarmament and Security Issues is headed by Olof Palme, former prime minister of Sweden and leader of that country's Social Democratic Party. The American member on the commission is former Secretary of State Cyrus R. Vance. The Soviet Union is represented by Georgi Arbatov, a Central Committee member and expert on American affairs.
In their report, entitled ''Common Security'' and published by Simon & Schuster in New York, the commission members argue that the United States and the Soviet Union now stand at ''parity'' in nuclear weapons.
While the Soviets hold the advantage in some parts of their nuclear arsenal, they say, the United States excels in others. They recommend a new strategic arms control treaty providing for ''sharp reductions and qualitative limitations resulting in essential parity at substantially lower and more stable levels of forces.''
Some commission members appear to be optimistic about the possibility of a 1983 arms control breakthrough in part because talks with Soviet officials have convinced them that the Soviets will be more forthcoming when it comes both to providing data on their military forces and agreeing on procedures for verifying agreements.
Barry M. Blechman, a former Carter administration official who worked as an expert for the international commission, said that under President Carter ''considerable progress'' had been made on the issue of verification in negotiations with the Soviets.
The international commission places considerable stress on limiting qualitative improvements in both nuclear and conventional weapons and recommends six interrelated steps:
(1) The reopening and rapid conclusion of nuclear test ban treaty negotiations, (2) an immediate halt to the militarization of space, including an end to work on antisatellite weapons, (3) resumption of US-Soviet talks on chemical weapons in an effort to ban these weapons altogether, (4) adherence by all nations to the nuclear Non-Proliferation Treaty, (5) placement of sensitive parts of the nuclear fuel cycle in international ''banks,'' where they can be controlled and managed, and (6) the opening of talks among supplier and recipient nations on regional agreements to restrict sales of conventional arms.
The commission warns that the world ''may be on the brink of a major new arms race in chemical weaponry.'' It notes that it has been estimated that if chemical weapons were used in densely populated Europe, the ratio of noncombatant-to-combatant casualties could be as high as 20 to 1.
''Moreover, the use of chemical weapons would blur the distinction between conventional and nuclear warfare,'' it says. ''This would increase the danger of one sliding into the other.''
The commission calls for a chemical weapons-free zone in Europe, beginning with Central Europe.
The agreement would include a declaration of the whereabouts of existing depots in Europe and means to verify their destruction, including on-site inspections. The training of troops in the use of chemical weapons would be prohibited.