President Reagan announced yesterday that he has given U.S. negotiators in the strategic arms reduction talks (START) at Geneva "new flexibility" in an effort to obtain long-sought agreement with the Soviet Union to restrict the two superpowers' intercontinental nuclear weapons.

Reagan said the new U.S. negotiating position in the START negotiations would allow each side to deploy more land- and submarine-based missiles, which the Soviets want, but would continue to limit the number of warheads they could carry to 5,000 for each side.

Reagan revealed few other details in an announcement that was designed, White House officials said, to avoid prompting a negative Soviet response before the revised U.S. proposal is submitted in Geneva. "There may be more than one way to achieve our objective of greater stability at reduced levels of nuclear arms," Reagan said in a speech to congressional leaders and administration officials in the Rose Garden of the White House.

"So I have instructed Ambassador Edward L. Rowny to make clear to the Soviet delegation our commitment to our fundamental objectives, but I have also given him the flexibility to explore all appropriate avenues for meeting our goals."

Asking for "measured flexibility" on both sides, Reagan said, "To the leaders of the Soviet Union, I urge that this new opportunity not be lost. To America's friends and allies around the world, I say that your steadfast support for the goals of both deterrence and arms control is essential to the future."

While the new U.S. position retains the administration's original limitation of 5,000 ballistic missiles warheads on each side, about one-third of each superpower's present nuclear arsenal, it abandons the limit of 850 missiles on each side originally proposed by Reagan.

He did not announce a new figure, but congressmen briefed by the president estimated that the United States now will seek a ceiling of between 1,100 and 1,200 missiles on each side at the START talks, which resumed in Geneva yesterday after a 10-week recess.

This would follow the recommendations of the President's Commission on Strategic Forces, which wants a higher missile limit while maintaining the ceiling on warheads to encourage both sides to move to smaller, single-warhead missiles that would be less tempting targets than multi-warhead missiles.

The president yesterday confirmed that he would extend the life of this group, better known as the Scowcroft Commission, which was due to expire on June 16. Administration officials said the commission would remain in existence at least until Jan. 1 while ways of continuing it or of naming a similar arms control advisory body on a permanent basis are explored.

Reagan's announcement followed several days of consultation with moderate congressmen of both parties, whom the president is counting on to continue supporting a principal Scowcroft commission recommendation that 100 new MX intercontinental ballistic missiles be deployed in existing Minuteman silos. Another test vote on MX development is expected in Congress next week.

Though Reagan's customary anti-Soviet rhetoric was muted yesterday, he did blame the Soviets for the failure of three previous rounds of START talks to make headway toward agreement.

"It's my judgment that these rounds have been useful and have permitted us to cover necessary ground," Reagan said. "However, due largely to Soviet intransigence, we have not yet made meaningful progress on the central issues."

In Geneva yesterday, Rowny, the head of the U.S. team, said he was "a little more hopeful" of some progress in the negotiations, which U.S. officials on both sides of the Atlantic emphasized will be conducted in as much secrecy as possible in an effort to make a breakthrough.

Reagan sidestepped one sensitive and complex issue yesterday and was conciliatory on another.

The first was the question of a nuclear "build-down," advocated by several influential congressmen, in which both sides would, generally speaking, remove two older weapons for each new one they deploy. Reagan, who is known to fear that it would be difficult to verify build-down, said the administration is continuing "high-priority work" on how this concept could be applied.

On the issue of throw-weight, the lifting power of missiles, Reagan was responsive to the concerns of congressmen that the United States take a conciliatory position and not insist directly on equal throw-weight.

Congressmen and some in the administration are concerned that excessive emphasis on equality of throw-weight would be a barrier to any agreement, since the Soviets are acknowledged to have a substantial advantage.

Reagan and a senior administration official who briefed reporters before the speech explained that throw-weight could be addressed "indirectly" by counting missiles and warheads.

This appeared to be a setback for Defense Secretary Caspar W. Weinberger, who had argued strenuously for strict equality in throw weight, and a victory for Secretary of State George P. Shultz, who favored the indirect approach.

"We believe, as does the Scowcroft commission, that stability can be increased by limitations on the destructive capability and potential of ballistic missiles," Reagan said.

"As a consequence, we will continue to propose such constraints which indirectly get to the throw weight problem while making clear to the Soviets are our readiness to deal directly with the corresponding destructive capability if they prefer."

Reagan's statement won cautious words of approval from moderate congressmen who are the potential swing votes on MX. Rep. Norman D. Dicks (D-Wash.), one of the House members briefed by the president Tuesday, said he found a "demonstration of stability" in Reagan's statement.

The flexibility of the administration apparently does not extend to combining the START talks with separate negotiations in Geneva designed to reduce the number of medium-range nuclear missiles based in Europe. Shultz told a news conference in Paris that he did not see "any need to merge these negotiations."