In moves that appeared to break off U.S. mediation efforts, Argentine Foreign Minister Nicanor Costa Mendez yesterday postponed a meeting with Secretary of State Alexander M. Haig Jr. and said the British attack on South Georgia island meant his government cannot deal with Britain at this time.

"For us, it means no negotiations with Great Britain for the moment," Costa Mendez told reporters outside his hotel here shortly after the State Department announced postponement of a meeting at which he and Haig had been scheduled to discuss a new American proposal for resolving the Falkland Islands dispute.

The Argentine minister charged that the British decision to attack while Haig was acting as a mediator between London and Buenos Aires "violated the general principles of international law while negotiations are on between two countries."

His attitude indicated that Haig's three-week exercise in intercontinental shuttle diplomacy had been thrown at least temporarily off course. However, the State Department made clear that the Reagan administration intends to try to get its peace-making efforts moving again before the crisis widens into further warfare.

In announcing that Argentina had asked to put off yesterday's meeting "in light of events in South Georgia," the department stressed that Haig had "lengthy conversations by telephone" with Costa Mendez after his arrival here yesterday afternoon. It added that, in these talks, Haig "made it clear that President Reagan believes every effort should be made to find a peaceful solution."

Earlier, after the first reports of fighting on South Georgia, the department had said: "The reported incident is further confirmation of the gravity of the situation and demonstrates anew the urgency of a diplomatic solution. The United States remains committed to that objective, and we will continue our ongoing efforts."

Although the announcement last night said Haig and Costa Mendez will talk again this morning, department spokesman Dean Fischer said it was not clear whether they would meet face to face or confer by phone. Fischer added that the discussions would be before this morning's scheduled start of a meeting of Organization of American States foreign ministers.

The meeting was called at Argentina's request to consider whether Britain is committing aggression against Argentina and whether the 21 active OAS members that have signed the 1947 Rio Treaty of mutual hemispheric defense should assist Argentina. The United States had opposed the OAS meeting, and Fischer said last night that Haig had not yet decided whether he will lead the U.S. delegation or delegate that task to OAS Ambassador J. William Middendorf.

What did seem immediately evident was that the South Georgia attack had overtaken and perhaps derailed the latest U.S. plan for resolving the dispute peacefully.

In discussions with British Foreign Secretary Francis Pym last Thursday and Friday, Haig had carved out a new series of proposals to settle the key questions of sovereignty over the Falklands, their interim administration after withdrawal of the Argentine forces that invaded on April 2 and negotiations to determine their future status.

In an intricate diplomatic maneuver designed to give Prime Minister Margaret Thatcher's government leeway to repudiate the proposals if they proved unpalatable in Britain, the British delegation pronounced them "unsatisfactory." But Pym also authorized Haig to relay them to Argentina and determine whether they might provide the basis for renewed talks.

According to U.S. sources, Haig and Costa Mendez were supposed to discuss this plan yesterday, and administration officials were known to have been hopeful that the meeting would pave the way for another Haig visit to Buenos Aires this week to pursue the plan further with President Leopoldo Galtieri's military government.

But by the time Costa Mendez arrived in Washington early yesterday afternoon, news of the South Georgia attack had triggered angry Argentine denunciations of Britain, and the minister failed to arrive at the State Department for the scheduled 4 p.m. meeting.

For the next 3 1/2 hours, while Haig remained closeted with key aides, department officials were unable to say whether the meeting would take place. Then, shortly before 7:30 p.m., Fischer came to the department press room to read the postponement statement.

A few minutes later, Costa Mendez, who is staying at the Embassy Row Hotel on Massachusetts Avenue NW, told reporters that his country was breaking off the indirect negotiations. Although he said the attack meant Britain and Argentina "technically" were at war, he added, "I think we can prevent the conflict yet."

He also would not say whether Argentina will ask the OAS to brand Britain as an aggressor and invoke diplomatic, economic or military sanctions against London. When a reporter asked if Argentina could expect anything more than "rhetorical support" from the rest of Latin America, the minister shot back: "This is a question I do not accept."