Suddenly the shuttle of foreign ministers is headed toward Washington, not the other direction, as the Falklands crisis enters a new and critical phase.
A possible casualty of the rapidly changing situation appears to be the US mediation effort, despite White House assurance that the mission of Secretary of State Alexander M. Haig Jr. continues.
British Foreign Secretary Francis Pym indeed is here to discuss London's latest Falkland proposals with Mr. Haig and other US officials.
But Argentine Foreign Minister Nicanor Costa Mendez is coming to Washington for a different purpose -- to expand the arena of negotiations to include the Organization of American States (OAS).
The government in Buenos Aires wants the OAS, at a special meeting of foreign ministers April 26, to consider sanctions against Britain under the aegis of the 1947 Rio de Janeiro mutual defense treaty.
This pact pledges its 21 active signatories, including the United States, to come to each other's assistance in the event of aggression from outside the hemisphere.
The Reagan administration opposes invocation of the treaty by Argentina, partly because Haig's mediation role would be undermined, and partly because the clouded issue of sovereignty over the Falklands makes the treaty's application unclear.
An armada of British warships, meanwhile, draws closer to the Falklands and a potential clash with Argentine aircraft or ships.
Against this background, Britain and Argentina reportedly are still far apart on an agreed solution to the Falklands problem, with the timing of sovereignty a key issue.
Britain, according to reports from London, demands immediate withdrawal of Argentine troops now occupying the islands and a restoration of British rule for a transition period.
Only the British flag would fly over the Falklands for an interim period, during which Prime Minister Margaret Thatcher's government would try to assure rights for the 1,800 islanders.
Negotiations on a transfer of sovereignty to Argentina then would commence, including protection of the rights of islanders who chose to remain.
The British fleet, meanwhile, would have turned around and steamed for home, after Argentine forces had left the islands and the British flag was run up.
One sticking point in all this, from the Argentine point of view, appears to be the question of control during the interim period.
The regime of President Leopoldo Fortunato Galtieri offers joint British-Argentine administration of the Falklands during a transition period, reportedly to the end of this year. The islands then would belong solely to Argentina.
Even during the period of joint administration, the Argentine flag would fly alongside that of Britain. President Galtieri is thought to be unwilling -- or powerless politically -- to take down symbol of Argentine sovereignty, even briefly.
Britain also reportedly wants more explicit guarantees from Buenos Aires of the rights of the Falkland Islanders, who are mainly of British origin, under Argentine rule.
An additional complication may be the status of South Georgia, a Falklands dependency about 1,200 miles east of Cape Horn. Taken by Capt. James Cook in 1775, South Georgia is claimed by Buenos Aires and was taken by Argentine troops after a battle April 3, a day after the Falklands seizure.
Mrs. Thatcher, speaking in London, said that South Georgia ''was administered as a matter of convenience through the government of the Falklands. Our title to it is quite different to that of the Falklands. It is extremely important to us.''
Some reports say that a detachment of the British armada has left the main fleet and is steaming toward South Georgia, with the aim of retaking the island from Argentina.