BRUSSELS, MARCH 3 -- NATO leaders wound up their first full-scale meeting in six years today with a show of unity that sidestepped differences on upgrading allied tactical nuclear weapons and gave broad endorsement to a U.S.-Soviet strategic nuclear arms treaty that would cut superpower arsenals of long-range nuclear weapons in half.

Speaking to reporters after the two days of meetings had ended, President Reagan promised that the U.S. commitment to protect Europe with nuclear and conventional forces would be maintained whatever happens at his upcoming Moscow meeting with Soviet leader Mikhail Gorbachev.

"All of us understand the absolute necessity of maintaining the credibility of our deterrent," Reagan said. "We will never trade that credibility away at the negotiating table and we won't give it away through neglect."

{Reagan returned to Washington Thursday night.}

But while Reagan praised the 19-point declaration agreed to by all 16 North Atlantic Treaty Organization nations except Greece, which has long favored a nuclear-free Europe, he said soon afterward that he had not read it.

At a picture-taking session before meeting privately with British Prime Minister Margaret Thatcher, the president was asked by reporters if he was pleased with the NATO communique.

"Haven't seen it," Reagan replied.

White House chief of staff Howard H. Baker Jr. came to the president's rescue, saying, "No, we saw it last night. No problems and it's very good."

"Yes, very good," Reagan repeated. "No problems."

Mirroring statements that have been made by Reagan, Thatcher and other western leaders, the communique took a positive view of changes initiated by Gorbachev.

"We have noted encouraging signs of change in the policies of the Soviet Union and some of its allies," it said. But it added that "we have to date witnessed no relaxation of the military effort pursued for years by the Soviet Union."

A key section of the declaration avoided the controversial issue of which, if any, allied forces will be "modernized" by saying that NATO will continue "a strategy of deterrence based upon an appropriate mix of adequate and effective nuclear and conventional forces, which will continue to be kept up to date where necessary."

The declaration restated the basic aims of the alliance, a favorite Reagan theme in recent weeks as he has sought to reassure Europe that superpower arms control treaties will not diminish the U.S. commitment to defend the continent.

"The presence in Europe of the conventional and nuclear forces of the United States provides the essential linkage with the United States strategic deterrent. . . . This presence must and will be maintained," the communique declared.

"A free, independent and increasingly united Europe is vital to North America's security," it continued. "The credibility of allied defense cannot be maintained without a major European contribution. We therefore welcome recent efforts to reinforce the European pillar of the alliance, intended to strengthen the transatlantic partnership and alliance security as a whole. The Atlantic Alliance cannot be strong if Europe is weak."

The compromise on modernization was crafted to commit NATO in general terms to preserve the credibility of its nuclear deterrent while satisfying West Germany's desire to avoid pledging now that it would deploy upgraded short-range nuclear weapons.

The communique did not deal directly with growing expressions of concern in the United States that Europe should pay a greater share of alliance defense costs.

When Secretary of State George P. Shultz was asked about this after the NATO meeting, he said, "Of course we feel that Europe should pick up more of the share," but he also criticized congressional cuts in the U.S. military budget.

The declaration praised the Intermediate-range Nuclear Forces treaty signed by Reagan and Gorbachev in Washington last December as "a milestone in our efforts to achieve a more secure peace and lower levels of arms."

It urged a 50 percent cut in superpower nuclear arms, the goal of the U.S.-Soviet negotiations under way in Geneva.

The leaders also gave broad endorsement to Reagan's objectives at the Moscow summit, saying, "We strongly support the efforts of the United States," and calling for "early and substantial progress with the Soviet Union on a full range of issues, including greater respect for human rights, arms control {and} a lessening of regional tensions."

The declaration said the "speedy and complete withdrawal of Soviet troops from Afghanistan," as promised by Gorbachev, would be an event of "major significance." Both Reagan and Shultz have said they expect Gorbachev to carry out his pledge to withdraw the troops.