In its latest post-Falklands effort to mend fences with Latin America, the United States has embarrassed and irritated its transatlantic ally, Britain.
With President Reagan preparing a visit this month to three Latin American countries, Washington has thrown its support behind an Argentine-sponsored resolution here calling for resumed negotiations over the Falkland Islands.
By doing so, the US has all but assured adoption of the General Assembly resolution which was expected, at time of writing, to be voted upon late Nov. 4. Also considered possible was a postponement of the vote.
Britain reportedly feels that it is too soon after it paid a heavy price in men and money to repossess the islands, for talks about the future of the islands to be resumed. It is also unhappy about specific aspects of the resolution.
British Prime Minister Margaret Thatcher personally appealed to Reagan to change his stand, but to no avail, according to reports. British diplomats here express their ''anger and dismay'' over what they consider to be ''a stab in the back.''
Western observers here feel that by supporting Argentina, the US may be sending the wrong signal to the military in Buenos Aires a second time. Says one European ambassador: ''The Reagan administration had been very eager to cozy up to the Argentine military all through 1981. They came to the conclusion that Washington was on their side and launched their assault on the Falklands. Now they may again believe that the US understands them and that their only mistake was to lose the war, not to start it.''
The Argentinian military - discredited as a result of its handling of the Falklands - has been pushing here for what it views as a moral victory to improve its image.
Under Brazilian advice, it toned down the wording of its draft resolution and included a reference to de facto cessation of hostilities as well as to the need to take into account the interests of island inhabitants.
These concessions were not, however, considered to be sufficient by the British, who insist that the wishes of the inhabitants - rather than their interests - must be taken into account. It also says that Argentina must completely renounce the use of force to regain control over the islands.
Debate on the resolution has made many General Assembly members uneasy.Many countries sympathize with the British view that Argentina was the aggressor eight months ago and that Britain is not yet prepared to resume talks with Argentina.
The US went so far as to lobby for a positive vote. But late reports indicated that most if not all of West European nations would abstain in the voting. Many, however, remain eager to renew trade with Argentina and thus considered seriously whether to vote for the resolution.