Secretary of State Alexander M. Haig Jr. is understood to have outlined to NATO allies today a three-stage U.S. plan for engaging the Soviet Union in talks to limit European-based, intermediate-range missiles.

At a closed-door meeting ot NATO foreign ministers, Haig promised that the Reagan administration will try to resume arms-limitation negotiations by the end of the year, according to sources familiar with today's developments.

The pledge, a response to allied concerns that could hinder the stationing of U.S. nuclear missiles in Europe -- is understood to call for preliminary discussions to be conducted through the Soviet ambassador in Washington, Anatoliy Dobrynin. These would be followed by more detailed talks between Haig and Soviet Foreign Minister Andrei Gromyko when they meet at the United Nations in the fall. If mutual agreement can be worked out at that time, actual negotiations would get under way shortly afterward.

Haig also revealed to the ministers a letter to Soviet President Leonid Brezhnev. Although the contents could not be ascertained and it was not immediately clear whether the letter dealt directly with the theater nuclear forces question, it was believed to be part of a concerted response by NATO leaders replying to letters sent them by Brezhnev recently in which he called for new detente initiatives on several fronts.

The timetable outlined today does not mean that the hoped-for talks automatically will fall into place. For that to happen, the Soviets will have to accept a negotiating framework agreeable to the United States and as Haig is known to have stressed today, the process will be undermined if the Soviets intervene in Poland or engage in other actions that Washington regards as unacceptable.

The U.S. move today comes at a time of concern over deepening antinuclear sentiment in West Germany and several other European countries, which has raised fears of possible reconsideration by the governments involved of accepting the missiles on their territory.Only last week, West German Foreign Minister HansDietrich Genscher indicated he would press Haig for "a clear signal for a time frame" for the arms-limitation negotiations for theater nuclear forces.

There was no immediate reaction from allied governments to Haig's remarks.

Haig is known to have argued within the Reagan administration in favor of accommodating European desires for early resumption of arms talks despite the more cautions approach advocated by the Defense Department, and today's pledge is likely to bolster his stature with NATO governments.

At issue is the decision made by NATO in December 1979 to deploy new American Pershing and Tomahawk missiles in Western Europe, where they would be able to carry nuclear warheads deep into Soviet territory. In a companion move, NATO also made a thus far unsuccessful offer to the Warsaw Pact to negotiate limits on these weapons.

Since then, growing pacifist sentiment, notably in West Germany, Italy, Belgum and the Netherlands, has raised fears of political reactions that could forestall deployment of the weapons in those nations.

For that reason, the West Europeans, led by West German Chancellor Helmut Schmidt, have been warning Washington that the entire deployment plan could be jeopardized unless they can demonstrate to their domestic constituencies that a good-faith effort has been made to negotiate with the Soviets on limitations.

However, the European pressures for a stepped-up effort ran against the grain of the Reagan administration's hard-line approach to Moscow. Reagan has rejected the Strategic Arms Limitation Treaty (Salt II) negotiated by the Carter administration, and the new American leadership has sought to link the prospects of talks to continued Soviet restraint in Poland and conformity to an international code of good behavior.

That made Haig, who served five years as commnder of NATO military forces, the key figure in working out the trans-Atlantic differences. The pledge he made on Reagan's behalf today underscored that he had won his fight to convince the president that failure to allay the Europeans' worries would imperil the missile modernization that is intended to be the backbone of NATO defenses for the years ahead.

In his remarks today, Haig is known to have coupled the U.S. promise with a dose of tough talk aimed at Moscow. He asserted there should be no misunderstanding about the move in any way signifying acceptance of Soviet conduct or a lessening of Washington's determination to meet Soviet challenges.

He noted that, by U.S. count, the Soviets presently have 1,040 missile warheads that could be used against Western Europe, while NATO will have only 572 such weapons to counter the Soviet threat by the time its missile deployment is completed in 1988.

For that reason, he added, the United States sees negotiations on theater nuclear forces controls as a possible means of restraining Soviet power