Secretary of State George P. Shultz, seeking to ease the anxiety and opposition of America's European allies, said today that the unratified SALT II strategic arms limitation treaty has ceased to be an effective vehicle for averting a U.S.-Soviet nuclear confrontation and should be replaced by other means.

Speaking at a meeting here of North Atlantic Treaty Organization foreign ministers, Shultz, in effect, confirmed to the Europeans that the United States does not consider itself bound to continue voluntary adherence to SALT II restraints on the size of the American nuclear arsenal.

His explanation of the decision announced Tuesday by President Reagan provoked what some participants in the closed-door meeting described as a "free-for-all" of heated debate and opposition from the allies concerned about the negative effects on Western European public opinion.

These sources, who asked not to be, identified, said the opposition was particularly strong from Britain, West Germany, the Netherlands and Belgium -- all countries that are deploying U.S. mediumrange nuclear missiles on their territory despite heavy domestic antinuclear pressures.

The sources added that reservations about the wisdom of the U.S. position were expressed by all the other members of the 16-nation alliance. One source said, "If there had been a vote, it would have been 15-to-1 against the United States."

However, the sources said, Shultz stood firm in insisting that the U.S. stance was necessary because of repeated Soviet violations of the SALT II restraints and changing assumptions about nuclear strategy. The sources said that Shultz expressed the belief that European fears will be allayed once the reasons for Reagan's decision are fully and clearly explained.

Underlying the controversy was the White House announcement Tuesday that Reagan would take two Poseidon submarines out of service this spring, thus keeping the United States under SALT II limits for intercontinental ballistics missiles.

But the administration added that in the fall, the United States would exceed SALT II limits by deploying new air-launched cruise missiles on B52 bombers. Although Reagan said that future U.S. moves would depend on Soviet actions, U.S. officials have described his decision as serving notice of the administration's intention to move away from SALT II.

Sources here said Shultz argued today that the United States could not permit itself to be bound by a treaty that was never ratified by the Senate and that was supposed to last only for a limited time.

He told the Europeans that Soviet violations were placing the United States at a strategic disadvantage and that new concepts of nuclear warfare made SALT II obsolete.

The participants said he acknowledged that in Europe, SALT II has become "a symbol" of superpower restraint whose continuation is considered necessary for the success of future U.S.-Soviet arms control talks.

But, the sources said, Shultz argued that the goal of avoiding nuclear warfare can be served better if the United States pursues a negotiating strategy aimed at achieving "mutual restraint" and "radical reductions" by the two superpowers.

He said that these goals can be purused through such avenues as future summit meetings between Reagan and Soviet leader Mikhail Gorbachev and intensified arms control talks. If such efforts move forward successfully, he said, the importance of SALT II will diminish in European opinion.

On the other side, the sources said, the Europeans countered that the U.S. decision would be seen in their countries as an American abandonment of restraint and reawaken fears that Reagan is not really interested in arms control.

Another topic that has been discussed intensively here involved the misunderstandings created by European criticism of the U.S. air strikes against Libya. Several European sources said public opinion in their countries remains hostile to the Libyan raids, and they warned that there is a major risk of the SALT II controversy widening the rift.