Washington — Increasingly the United States, caught in the middle of the Falklands crisis, is under pressure to side either with America's closest ally or with a nation that until recently was a pariah in Washington.
So closely are the fortunes of Britain and the US entwined that the intelligence services of the two nations automatically share more information with each other than either does with any other nation.
Argentina, by contrast, cannot under US law receive any form of military aid or training from the United States because of human rights violations by successive regimes in Buenos Aires.
For such aid to begin to flow, President Reagan would have to certify to Congress that Argentina has improved its observance of human rights. Mr. Reagan has not done so. Even a minuscule $50,000 in Reagan's 1983 budget earmarked for US training of Argentine forces will not be spent unless such certification is made.
Powerful voices in Congress and the press are urging the White House to recognize the inevitable - that, if mediation by Secretary of State Alexander M. Haig Jr. fails, US interests and sympathy lie primarily with Britain.
But the situation is not that simple. Reagan and Haig have been trying to drum up Latin American support, including that of Argentina, for combating leftist influence in Central America, especially El Salvador.
A US tilt toward Britain in the Falklands dispute would make it hard, if not impossible, for some Latin American governments to reward Washington with active backing for its anti-communist campaign.
So far Reagan administration officials insist there is no tilt, because Mr. Haig's mediation effort, though disrupted by the British seizure of South Georgia Island, is alive.
''While the situation is increasingly difficult and time is truly running out ,'' said President Reagan Monday, ''we remain determined to do all we can to help Britain and Argentina solve their differences, without further conflict.''
British Ambassador to Washington Sir Nicholas Henderson, speaking after British troops recaptured South Georgia, said the US has ''a crucial role'' to play in pressing for a peaceful settlement.
If the current session of the Organization of American States (OAS) contents itself with a general expression of support for Argentina, and if British troops do not attack the Falklands themselves, the stage might be set for Haig's resumption of mediation.
In that case, some sources say, a premature tilt by the US toward Britain could torpedo the Haig mission.
A declaration of economic sanctions against Britain by the OAS, on the other hand, would put the US in a difficult position.
Influential lawmakers like Sens. Henry M. Jackson (D) of Washington and Daniel Patrick Moynihan (D) of New York urge the US to join the Common Market's declaration of sanctions against Argentina.
The US is not treaty-bound to join the Common Market's trade boycott of Argentina. But a two-thirds vote by member nations of the Rio treaty to invoke economic sanctions against Britain would be binding on the US under the terms of the 1947 Rio pact.
To secure sanctions, Argentina must garner 14 approving votes among the treaty's 21 active members. (Cuba, 22nd signer of the treaty, was barred from the OAS in 1962.)
US officials, led by Haig, reportedly will try to head off any vote on sanctions at the current OAS meeting. Should a vote come, the United States would lobby against a majority for Argentina.
Pro-British sentiment in Congress and among many Americans is bolstered by the fact that Britain's Common Market and European NATO partners solidly back Prime Minister Margaret Thatcher's Falklands policy.
Aversion to Argentina's record on human rights runs high among liberals in Congress, who also note that Buenos Aires stepped up its grain sales to Moscow during former President Jimmy Carter's grain boycott of the Soviet Union.
The USSR now takes an estimated 70 percent of all grain exported by President Leopoldo Galtieri's Argentine regime.
Still to be heard, as the Falklands drama unfolds, are the views of 15 million Americans of Hispanic origin, concentrated in the US Southwest and New York City.