Washington — The United States and Argentina have split on what to do next in the swiftly changing crisis over the Falkland Islands.
Against US wishes, Argentina has won approval from the Organization of American States (OAS) to convene a special meeting of hemispheric foreign ministers as a first step toward invoking the 1947 Rio treaty.
The United States, one of three countries abstaining in the 18-0 OAS vote Tuesday, had hoped to confine negotiation over the British-Argentine dispute to the continuing mediation efforts of Secretary of State Alexander M. Haig Jr.
''The United States is deeply disturbed by the implications of (this) action, '' said US Ambassador to the OAS William Middendorf.
Reminding OAS delegates that the Rio mutual defense treaty implies collective security measures, Mr. Middendorf said the vote ''inevitably casts this organization in a unhelpful, confrontational light.''
Broadening the scope of diplomacy to include 22 foreign ministers would appear to make it harder for Britain and Argentina, through the efforts of Mr. Haig, to reach a compromise settlement.
Meanwhile, at an impromptu Tuesday press conference, President Reagan praised Haig for doing a ''magnificent job,'' said it was ''very difficult'' to know how matters would turn out and noted that British Foreign Secretary Francis Pym was coming to Washington to continue discussions.
On balance, it now appears harder for the Reagan administration to maintain the neutral role it has tried to stake out for itself.
Like so much else that is murky in the Falklands crisis, the treaty Argentina wants to invoke raises as many questions as it answers.
So long as the US retains a mediating role, Washington has a chance to avoid complications that might arise from application of the Inter-American Treaty of Reciprocal Assistance, as the Rio pact is formally known.
The US, along with 21 other signatories, agrees that ''an armed attack . . . against an American state shall be considered as an attack against all American states.''
That sounds explicit enough. But relevance of the treaty to the Falkland Islands crisis could keep international lawyers busy for a long time.
Sovereignty and aggressorm are key words in the drama now being played out, as Argentina appeals to treaty members to consider sanctions against Britain, whose fleet steams closer to the Falklands daily.
When the Rio treaty was signed in 1947, Argentina attached a reservation challenging Britain's claim to sovereignty over the Falkland Islands and their dependencies, the South Georgia and South Sandwich Islands. Argentina bases its right to occupy the islands on an uninterrupted claim to sovereignty dating back to 1833, when British forces occupied the islands, ousted the Argentine garrison , and raised the British flag.
''Argentina is prepared for peace because it wants peace,'' Argentine Ambassador to the OAS Raul Quijano told the special OAS meeting Tuesday in Washington. ''But it is not prepared to accept that its soil, which is also the soil of the Americas, be the object of a new unlawful seizure by force of arms.''
Foreign ministers of signatory states are designated by the treaty as an ''organ of consultation,'' empowered by a two-thirds vote to decide on a course of joint action. Sanctions thus voted by the foreign ministers are binding on all signatories, except, says the treaty, that ''no state shall be required to use armed force without its consent.''
Argentina values the mediation efforts by Secretary Haig, said Mr. Quijano, adding that Buenos Aires's request for OAS action should not be construed as an indication that Haig's mission has failed.
Nonetheless, said the Argentine diplomat, the approaching British armada poses a threat ''not only to Argentina, but to the entire American continent and endangers the region's peace and security.''
Quijano said his government seeks only the special meeting of foreign ministers at this time, not the immediate invocation of sanctions against Britain. This meeting currently is scheduled for April 26.
If Argentina's claim to sovereignty was recognized in international law, any British effort to wrest the islands away from Argentine troops presumably would constitute an ''armed attack against an American state.''
But the US carefully has steered clear of coming down on one side or the other in the British-Argentine sovereignty dispute.