Secretary of State George P. Shultz, defending President Reagan's decision no longer to be bound by the SALT II strategic arms limitation treaty, said today that the United States is "shifting gears" to emphasize maintenance of a credible nuclear deterrent.

"The United States has a responsibility to itself and its allies to maintain the effectiveness of our deterrent capability," Shultz said. "That's what the president has determined to do."

Characterizing the SALT II provisions as "so-called restraints" that are "obsolete, unratified and being violated by the Soviet Union," Shultz said: "We put all that behind us, and we say that there can be a de facto form of mutual restraints."

Shultz spoke at a news conference ending a meeting here of North Atlantic Treaty Organization foreign ministers. The two-day meeting was dominated by debate about U.S. determination to pursue a controversial new direction in arms control despite the unanimous objections of Canada and the West European members of the 16-nation alliance.

In the private discussions here, they are known to have objected to Shultz's argument that effective arms control can best be achieved by pressing the Soviet Union to negotiate radical cuts in nuclear weaponry rather than by following what he called "the technicalities" of SALT II.

The Europeans countered that in their countries, adherence to SALT II is widely regarded as a symbol of American restraint and sincerity in seeking meaningful arms control agreements. The allies fear that U.S. abandonment of its voluntary compliance policy will erode American credibility in Europe and reawaken strong antinuclear sentiment throughout the NATO countries.

"You have to do things that must be done," Shultz said in response to questions about the allies' concerns and an anticipated, angry reaction from the Soviet Union to what some European delegates called "the signal of Halifax."

The Europeans seemed particularly anxious to contain the impression that the Reagan administration's overriding of their objections signals a serious rift within the Atlantic Alliance. As a result, their public statements today avoided criticism of the United States and emphasized unity.

"The West's case must be presented with solidarity and conviction to the Soviet Union," British Foreign Secretary Geoffrey Howe said.

But the other ministers left no doubt about their unhappiness at the U.S. decision and expressed hope that Reagan's promise to take future Soviet conduct into account might induce him to continue at least partial compliance with the SALT II restraints.

Canadian External Affairs Minister Joe Clark, summing up the mood of the other NATO members about these two points, said:

"We believe it is in the interests of arms control that the limits of SALT II be respected . . . It's premature to put it in a coffin until there's a corpse."

Shultz, seeking to minimize talk of an alliance rift, noted:

"In diplomatic lingo, frankness is a synonym for criticism. At the opening of this session, our president, [NATO Secretary General] Lord Carrington, called for frank discussions. We had a lot of it."

Stressing the administration's conviction that its course is correct, Shultz said he had the impression that the Europeans are more concerned about "the imagery than the content" of Reagan's decision. He added:

"It is most important that we do the right thing. We must not do something because it sounds better."

In his public remarks, Shultz made essentially the same arguments that participants said were the basis of his presentation to a secret meeting of the ministers on Thursday.

He stressed that, in addition to Soviet violations, the SALT II treaty was never ratified by the Senate, was never meant to be of indefinite duration and calls for restraints based on the number of launchers possessed by the United States and the Soviet Union. In the U.S. view, he said, "the correct unit of count" for determining limits on the two countries' arsenals should be the number of their nuclear warheads.

Shultz was not specific about what he meant by "de facto mutual restraints." Asked what the United States would regard as an appropriate substitute system of restraints, he noted that Reagan, in announcing his decision, had listed several areas of nuclear weapons production and deployment where the United States would show restraint. He also repeated that Soviet conduct would be taken carefully into account in future U.S. nuclear planning and decision-making.

"We want to get away from the technicalities of what this unratified and increasingly obsolete treaty calls for and into the realities of what it takes to have a credible deterrent with the Soviet Union," Shultz said.

Most importantly, he asserted, the Soviet Union should start negotiating more seriously in the Geneva arms control talks and "get on with the task of what this business is all about: getting the numbers of nuclear weapons down through agreement on radical reductions."

Shultz said he did not think the new U.S. position should prejudice hopes for a summit meeting this year between Reagan and Soviet leader Mikhail Gorbachev. He said the United States continues to believe in the importance of a summit, but he added: "We can't make predictions about what the Soviets might do."

In a related action, the NATO ministers announced that they are forming a task force to further conventional arms-control negotiations.