NATO foreign ministers will begin their spring meeting Thursday with European members worried that the United States might endanger arms control by halting its compliance with SALT II limits on nuclear weapons and pushing ahead too aggressively with President Reagan's Strategic Defense Initiative.

Allied concern that these two issues could generate a hostile public backlash in Western Europe and further impede progress in the Geneva arms talks will be the principal issue confronting Secretary of State George P. Shultz at this session of the 16-nation North Atlantic Treaty Organization.

However, concern about the subject that last week threatened to dominate the meeting -- Reagan's pending decision on whether the United States should continue abiding by provisions of the unratified SALT II agreement -- appears to have abated considerably in the past two days.

A senior U.S. official said tonight that Reagan still has not decided what course he will follow and is waiting for Shultz's report on the sentiments expressed by the allies. The official said the first of these reports, focusing on Shultz's talks today with British Foreign Secretary Geoffrey Howe and West German Foreign Minister Hans-Dietrich Genscher, was sent to Washington tonight, and a more detailed report will follow Thursday.

The official said the Europeans had expressed understanding for the difficulties that Soviet violations of the SALT II provisions have caused for U.S. military policy. He added: "It is a matter of some delicacy. We don't want to ignore the Soviet violations. But we also don't want to damage the prospects for future negotiatons" with the Soviet Union.

Despite the U.S. official's insistence that Reagan is still making up his mind, sources within various European delegations said they believe Washington now is on notice that a blanket U.S. refusal to respect SALT II limits will have severely divisive effects on the alliance and give the Soviet Union a propaganda advantage with which to reignite sentiment against NATO's nuclear strategy.

As a result, the sources continued, the allies now seem relatively confident that Shultz will hint strongly at a compromise acceptable to European opinion. Specifically, they said, the Europeans expect Reagan to choose the so-called "gray-area approach" that will allow the United States to contend that it is in compliance with the spirit if not the strict letter of SALT II provisions.

After a meeting Monday of the National Security Council, U.S. sources said the United States was unlikely to dismantle a nuclear-armed Poseidon submarine when the new Trident submarine Alaska enters service. Full compliance with the SALT agreements would require dismantling of the Poseidon's firing tubes within six months.

However, Shultz is understood to have argued that the United States could comply with the spirit of the agreements by removing the Poseidon's nuclear missiles and putting it in indefinite drydock.

The Europeans say they now expect that Shultz will make clear in his secret talks here that Reagan's decision, which is to be announced Monday, will follow the general outlines of that plan.

The other issue with which Shultz will have to contend is European concern about the Strategic Defense Initiative, a research effort to build a high-technology shield against Soviet nuclear missiles.

Reagan's claims that such a program could make nuclear war obsolete have made almost no headway against European fears that it will mark the start of a new arms race in outer space, lessen U.S. willingness to defend Western Europe from nuclear attack and give the Soviets an excuse for refusing to negotiate in Geneva on reduction of nuclear missiles.

On the surface, most of the NATO allies have gone along with the SDI to the extent of endorsing "research" into such defenses. But continued distrust in Western European public opinion has forced many allied governments to seek to deemphasize the subject's prominence and get it out of the realm of constant public discussion.