Argentine Foreign Minister Nicanor Costa Mendez accused British Prime Minister Margaret Thatcher tonight of "absolute intransigence" as United Nations-sponsored peace negotiations appeared to be collapsing.

Costa Mendez, summoning reporters, said U.N. Secretary General Javier Perez de Cuellar was continuing his efforts to arrange peace in the South Atlantic and that Argentina "is ready to continue negotiating."

Diplomatic officials said that the talks appeared doomed, however, and military officials said a British attack on Argentine forces occupying the Falkland Islands was considered imminent.

A presidential palace spokesman confirmed that the president, Gen. Leopoldo Galtieri, spoke with Perez de Cuellar by telephone today in an effort to revive the talks.

Costa Mendez said, "The only obstacle is the absolute intransigence of Mrs. Thatcher and hers will be the responsibility if the conversations fail."

Costa Mendez said the U.N. Security Council had held an informal meeting today to consider the situation, and that he was prepared to travel to New York "at any time." Asked about reports that Britain had earlier placed a 48-hour deadline on reaching a peaceful settlement, Costa Mendez replied, "Argentina does not accept ultimatums from anyone."

Foreign Ministry officials had earlier maintained today that Argentina was continuing to present "new ideas" in the U.N. negotiations, and avoided saying that a document submitted to Perez de Cuellar last night rejected Britain's final position.

But behind the emphasis on British intransigence and Argentina's new ideas lay a rejection of the formula proposed by British representative Anthony Parsons on Monday after a weekend of consultations in London, government officials here conceded.

Gustavo Figueroa, cabinet chief in the Foreign Ministry, said in a radio interview this morning that Britain had no "willingness to negotiate," adding, "the defender of Argentine interests is going to be Gen.Mario Menendez," the commander of the 7,000 to 10,000 Argentine troops reported to be positioned on the Falklands.

Figueroa said that in the event of a new upsurge of fighting, Argentina's next appeal could be made directly to the United Nations Security Council. Other government officials here speculated today that Perez de Cuellar himself might soon turn to the Security Council to call on both Britain and Argentina to cease military action.

The Argentine military command announced early this evening that two British Sea Harriers bombed an area seven miles from Stanley this morning, but gave no further details. The military command said the attack was the day's only military action in the Falklands.

Reports by government-designated Argentine "war correspondents" from southern port cities and bases today indicated increasing preparations for an attack.

An account from the key coastal city of Comodoro Rivadavia by the news service Noticias Argentinas said the "air bridge" of Argentine planes carrying supplies to the Falklands had been stopped. The interruption of this air route, the only means Argentina has to resupply its troops on the islands, has been a principal objective of the British task force in repeatedly bombing Argentine airfields on the island.

Another report from the operation zone in the morning paper La Nacion said that Argentine air operations over the South Atlantic--including a possible massive air attack on the British fleet threatened by Air Force Commander Basilio Lami Dozo--had been impeded by several days of bad weather.

The military government also issued a decree tonight prohibiting the sale or transfer of British assets in Argentina and creating a commission to supervise the management of British holdings. Officials explained that the measure was designed to prevent British firms from closing and leaving the country as tensions increase.

In reviewing the 12-day-old talks conducted by Perez de Cuellar in New York, Argentine officials stated here that a solution had been thwarted from the beginning by the inability of Britain and Argentina to discuss a cease-fire without simultaneously resolving the most difficult issues in the crisis--sovereignty and the rights of the 1,800 Falklanders.

This impasse, officials said, resulted because Argentina, while willing to agree to an unconditional cease-fire, would not agree to withdraw its troops from the islands unless the basic issue of Argentina's claim to the Falklands was resolved.

Britain, at the same time, reportedly would not agree to even a temporary cease-fire unless it was linked to the withdrawal of Argentine troops.

Thus, while the U.N. negotiations were technically designed to arrange a cease-fire and direct negotiatons between Britain and Argentina, any agreement that is signed would inevitably involve an at least implicit settlement of the underlying issues, according to Argentines here.

Argentina is no longer attempting through the negotiations to obtain an explicit guarantee of sovereignty, officials here said. Instead, the government believes that if the direct negotiations between Britain and Argentina have a fixed time period and can be narrowed to specific issues, Argentine sovereignty will be the only practical result.

For Argentina, narrowing the negotiations largely means ensuring that they are not expanded beyond a simple choice between Argentine sovereignty and continued British colonialism. For that reason, the focus of the U.N. talks has shifted from sovereignty to the rights of the island residents and the conditions of the temporary U.N. administration for the islands proposed by Perez de Cuellar, sources here explained.

Rather than attempting to attach conditions to the negotiations, Argentine officials see their main objective as preventing Britain from introducing conditions to an agreement that would give the interests of the island's residents, known to be hostile to Argentine rule, equal or greater weight than those of Argentina.