President Reagan yesterday dispatched Secretary of State Alexander M. Haig Jr. on an urgent mission to London and Buenos Aires in hopes of keeping the crisis between Britain and Argentina over the Falkland Islands from exploding into warfare.

Haig was scheduled to leave early this morning for London on the first leg of the search for a peaceful resolution of the confrontation that began last Friday when Argentine military units forcibly occupied the islands that have been under British control for 149 years.

The decision to have Haig explore with British and Argentine leaders the chances for averting an armed clash in the South Atlantic was made by Reagan yesterday morning following a White House meeting with his national security advisers. Reagan acted just before his departure for a five-day "working holiday" in Jamaica and Barbados.

On his arrival in Jamaica, the president refused to answer questions about Haig's trip. But White House officials noted that the secretary had received invitations from both governments, and deputy chief of staff Michael K. Deaver said, "I don't think he would have gone if there weren't some signs of hope."

Haig had been scheduled to accompany Reagan to the Caribbean. His sudden shift to a mission that will take him first to London and then another 8,000 miles to the Argentine capital underscored anew the U.S. concern about heading off a potential armed clash between two governments whose friendship the administration regards as vital to U.S. policy goals in Europe and Latin America.

The secretary, whom State Department sources said is tentatively expected to return here around Sunday, will be working in a context of increasing tension. Prime Minister Margaret Thatcher's British government announced yesterday that it is instituting a blockade spanning 200 miles around the Falklands effective 11 p.m. EDT Sunday.

However, White House and State Department officials publicly insisted that Haig's aim, at this juncture, is to hold "preliminary discussions" with British and Argentine leaders. They pointedly refused to accept the term "mediation" as descriptive of Haig's mission, and they said "no" to questions about whether he is carrying any specific proposals for a solution.

In summarizing the purpose of Haig's trip, White House communications director David R. Gergen said: "The important thing is to avoid any further use of force. This is one more step in a process we hope will lead to a peaceful resolution."

Despite this cautious stance, some administration sources said privately that Haig will have what one called "some rough-sketch American ideas" about how the impasse over control of the Falklands might be broken. But, the sources stressed, these proposals are tentative and subject to modification after Haig finishes his consultations that will include talks with Thatcher and her new foreign secretary, Francis Pym, and almost certainly also with Argentine President Leopoldo Galtieri.

Haig conferred separately on Tuesday with Argentine Foreign Minister Nicanor Costa Mendez and British Ambassador Sir Nicholas Henderson, and the sources said he would not have undertaken the mission without assurances that both governments want to avoid a confrontation and are willing to show more flexibility than has been evident until now in their respective attitudes.

Specifically, Argentina has said that its claim to sovereignty over the Falklands must be recognized and that negotiations over other issues like the future status of the islands' approximately 1,800 inhabitants must be conducted while the territory remains under Argentine control. Britain has countered that it will not negotiate until the Argentine occupation forces are removed.

The degree to which each is willing to retreat from these positions is still not clear, the sources said. They added, though, that in the last two days the United States has detected sufficient signs of flexibility to make it worthwhile for Haig to probe further.

In particular, Argentina's military government is thought to have miscalculated the depth and vehemence of the British response and to have overestimated the degree of backing it would receive from the rest of Latin America, other Third World countries and the Soviet bloc. As a result, the Galtieri government is now regarded as potentially more receptive to a solution that would permit it to back away gracefully from a situation where it would find itself isolated and facing an armed clash that would have disastrous effects for the already embattled Argentine economy.

There has been considerable speculation about a solution that would tie together an Argentine withdrawal, an acknowledgment of Argentina's sovereignty over the Falklands and an agreement for continued British administration for a specific period of years. There also has been talk about this being done through some sort of "lease" arrangement that would include a sharing of any oil or other mineral resources that might be found in the waters around the islands.

In a related development yesterday, the Defense Department denied as "completely untrue" an ABC television news report Tuesday that a U.S. SR71 Blackbird reconnaissance plane overflew the Falklands before and after the Argentine invasion to gather intelligence that was shared with Britain.