U.S. Secretary of State Alexander M. Haig Jr. left Buenos Aires for Washington today carrying a new "working document" for settling the Falkland Islands crisis as Argentina sought to strengthen its diplomatic position by announcing it would seek to invoke an inter-American defense treaty against Britain.

After four days of tense, marathon meetings between U.S. and Argentine officials, Argentine sources confirmed late tonight that Argentina had made several significant concessions from its earlier negotiating position in the 17-day-old dispute with Britain over the South Atlantic islands.

The military government here is still seeking, however, some kind of assurance of eventual Argentine sovereignty over the islands, the sources said, adding that neither the United States nor Britain is willing to meet that condition.

It was also unclear whether Argentina's new position for an interim settlement of the conflict would be acceptable to Britain. Sources confirmed reports appearing in the Argentine press that Buenos Aires was now willing to allow a joint administration of the islands that would give Britain and Argentina equal status, with both flags flying over the storm-swept territory, until negotiations established its future government.

In Washington, American officials privately expressed the view that Haig's return might force both Argentina and Britain to take a new, hard look at what each country would accept to avoid war, Washington Post correspondent Michael Getler reported.

A number of officials expressed the opinion that it was best at this point to let both sides--but especially Argentina--"sweat a little," as one put it. Sources here said the decision for Haig to return was reached mutually by Haig and the White House.

In an apparent effort to increase pressure on both Britain and the United States, Argentine Foreign Minister Nicanor Costa Mendez announced hours after Haig's departure that Argentina would move to invoke the Rio Treaty of 1947 in the Organization of American States. Officials said the action was expected when the OAS convenes for a special session Tuesday in Washington.

Invocation of the treaty, which has been strongly opposed by the United States, would seek to apply Article 6 of the Rio Treaty providing for the United States and 21 other American nations to hold a special consultation in the event of "any . . . fact or situation that might endanger the peace of America."

Haig, who met with Argentine officials for a total of more than 25 hours beginning last Friday, publicly warned yesterday that "there are a number of complications" that make an invocation of the treaty inappropriate. He cited the longstanding defense ties between the United States and Britain.

Argentina, however, does not intend to immediately call for a condemnation of Britain or sanctions by the treaty members, according to political sources here close to the military command. Such measures would require a two-thirds vote of the treaty's signatories. Instead, the sources said, Argentine officials would seek only a majority of votes needed to hold the special consultative session.

In Washington, State Department officials said they would have no comment on Argentina's plan to invoke the treaty. The United States was understood to have warned Argentina that it would oppose the move at the OAS meeting.

The Argentine announcement of the step, which it has threatened to take ever since the early days of the crisis, came after Haig had left Buenos Aires with what he described as "a further identification and refinement of the Argentine position."

Costa Mendez indicated the new Argentine stance by saying that Argentina "foresaw the possibility" of an interim arrangement during which the competing claims of Argentina and Britain over the islands could be negotiated.

Haig, who conducted the long talks here with Costa Mendez and the Argentine military junta in a seemingly calculated atmosphere of tension and uncertainty, won agreement for a temporary administration of the islands that would include British participation, sources here confirmed.

In the past, Argentina has refused to accept British participation in any temporary administration of the islands, which it calls the Malvinas. Argentina has announced its annexation of the islands and has insisted on Argentine superiority in any governmental arrangements.

The reported change in the Argentine position over the weekend indicated that the military government decided it could no longer make such stringent demands in the face of an approaching British fleet threatening to involve Argentina in a potentially disastrous war.

Argentine sources also said, however, that Argentina still wanted an assurance from the United States and Britain that the negotiations would have a fixed term and would ultimately lead to Argentine sovereignty. "It could be in one year or five years or 10 years, but we have to have a guarantee of sovereignty," said one informed Argentine political source.

Haig, in a formal statement at Buenos Aires Ezeiza Airport, said he was "more convinced than ever that war in the South Atlantic would be the greatest of tragedies, and that time is indeed running out."

"We have now finished this stage of our work" in Buenos Aires, Haig said, adding, "I am making the results available to the British government." The secretary has traveled more than 28,000 miles during 12 days of shuttling among London, Buenos Aires and Washington.

Argentina has claimed the islands 400 miles off the southern coast since 1816, but the British seized them in 1833. Since ending 149 years of British control over the archipelago with an armed invasion April 2, Argentina has made the island's sovereignty into a national rallying cry.

Correspondent Getler in Washington added:

Officials here who have been watching the situation closely said that, under Haig's prodding, the Argentine government had made a number of "rather intricate changes" in a basic proposal for settling the crisis that had been presented to Haig shortly after he arrived in Argentina last Thursday.

These officials cautioned that, in their view, the changes do not add up to cause for optimism or belief that a breakthrough was near. Rather, they said some specialists believe the changes may be enough to at least keep discussions going.