Secretary of State Alexander M. Haig Jr., after assuring Argentina that U.S. assistance to Britain will not affect Washington's peacemaking role in the Falkland Islands crisis, announced yesterday that he will return to Buenos Aires today with some "new ideas" for averting a clash.

The decision to have Haig resume shuttling between Buenos Aires and London came as the secretary sought to smooth over the idea that the United States is tilting toward Britain with communications and intelligence help and as President Reagan, reacting to news reports that the Soviet Union is aiding Argentina, warned Moscow to "butt out" of the crisis.

However, the White House later acknowledged that Reagan's remark was a spur-of-the moment comment based solely on unconfirmed press reports and that U.S. intelligence agencies have no information to confirm that the Soviets have passed intelligence information to Argentina.

The president's comments temporarily added a new note of confusion to the tense effort to prevent the Falklands dispute from exploding into warfare. But administration sources said privately their main concern was that reports, which began circulating Tuesday night, about U.S. aid to Britain would shatter Argentina's faith in Haig's impartiality and cause President Leopoldo Galtieri to break off the secretary's mediation efforts.

Haig was known to be extremely upset both about the reports and about the willingness of unidentified administration officials to confirm them to the press.

The administration's initial response yesterday was to impose a ban on statements by U.S. officials about any aspect of the Falklands situation. According to administration sources, Haig also spent considerable time in telephone contact with Buenos Aires to reassure the Argentines of U.S. evenhandedness and good faith and to sketch some new proposals for bridging the gulf between the Argentine and British positions.

Finally, late yesterday afternoon, Haig appeared before reporters at the State Department to read a statement in which he said, "The leaders of both countries have assured me and in turn the president, again today, that they are prepared to go on working with us to reach a peaceful solution."

He added that, based on the "new ideas" developed in his talks Monday in London and by phone with Buenos Aires yesterday, "The Argentinians have invited me to return to Buenos Aires. I propose to do so tomorrow Thursday ."

Then, without referring directly to the aid the United States has provided to the British naval force steaming toward the Falklands, Haig sought to spell out Washington's intention to approach the crisis in as evenhanded a manner as possible. He said:

"The United States has viewed its role as that of assisting the two sides in finding a peaceful solution. Our ability to do this is based on our longstanding relations with both the United Kingdom and Argentina. We have been careful to maintain these relationships in order to preserve our influence with both governments. Failure to live up to existing obligations--or going beyond them--would obviously jeopardize our ability to play the role both countries wish us to perform.

"Since the onset of the crisis, the United States has therefore not acceded to requests that would go beyond the scope of customary patterns of cooperation based on existing bilateral agreements. That will continue to be our stand while our efforts are under way."

Underlying this carefully phrased language is the fact that Britain is the closest of American allies. Intelligence and military cooperation between the two countries has been very close, partly through tradition and partly through specific agreements, for many years. Thus, U.S. officials explained, it is virtually impossible to separate British access to much American information and facilities.

The new ideas Haig will carry to Buenos Aires were being kept secret, but some U.S. officials said they involved variations on past suggestions that would bring about an Argentine withdrawal from the Falklands and a turnaround of the British naval force in exchange for some kind of neutral authority--perhaps involving American, Canadian or Latin American forces--temporarily assuming administration of the islands.

Reagan's admonition to Moscow to "butt out" came when he was asked about some news agency and television reports that claimed Moscow was providing intelligence information on British fleet movements to Argentina.

"That has been reported and evidently is established. And I think that it's a--I'd like to see them butt out," Reagan said. When asked if this meant he was confirming the press reports, the president responded: "No. No. I said all I know is what I've heard and read . . . . "

Later in the day, White House deputy press secretary Larry Speakes told reporters he had talked personally with Reagan and confirmed that the president was "talking about press reports" and not about any private U.S. intelligence information about what Moscow was doing.

Officials in the White House, Pentagon and State Department all said privately yesterday that there is no evidence thus far that Moscow has supplied intelligence information on British fleet movements to Buenos Aires.

It has been clear for several days, these officials say, that Soviet reconnaissance planes and fishing trawlers equipped with intelligence gear have been tracking the fleet, which is not unusual because the British have one of the biggest navies in the North Atlantic Treaty Organization. But officials say there is nothing yet in intelligence reports to verify that Moscow has taken the further step of supplying that data to the Argentines.

Pentagon sources also said that U.S. intelligence thus far had nothing to confirm a report from London yesterday that two Soviet Echo-class submarines were now patrolling in the South Atlantic.

Officials explain that while Moscow might eventually help the Argentines with information, the British fleet was still too far away from the Falklands for that information to do much good and that it would not be easy for the Soviets to transfer information in a timely fashion even if they wanted to because the fleet is constantly moving around.