Argentina's military leadership today stuck to its strategy of wearing out British forces on the Falkland Islands with its Air Force, and, amid reports of continued heavy losses, indicated it would quickly look abroad for military support if its own forces grew thin.

With Argentine warplanes reported to be striking at British ships in the channel between East and West Falkland Islands for the third day since Friday, military officials reported that they were successfully encircling British forces and denying them crucial supplies.

The military's Joint Chiefs of Staff issued a communique late tonight confirming for the first time that British forces had succeeded in establishing a beachhead on East Falkland measuring about six square miles that was being defended by 2,000 men.

The communique added that Argentine planes, attacking British positions on the ground as well as transport and supply ships, had damaged one troop ship and one frigate. It said two Argentine planes had been shot down during today's fighting.

An earlier communique also said that British Harrier planes had bombed an area near Port Stanley without causing damage, and that one British plane had been shot down.

Defense Minister Amadeo Frugoli, the only high government official to speak out on the military situation today, said in a radio interview this morning that British troops near San Carlos on the northwest end of East Falkland Island "are surrounded."

Military analysts and officials quoted in reports here indicated that Argentina's strategy was to cut off invading British forces in East Falkland from the rest of the island with troops, and sever their sea-based supply lines by launching massive air attacks whenever British ships entered the sea channel leading to San Carlos. Today's communique contained the first report of air attacks on British ground installations, which are fortified with sophisticated antiaircraft defenses. The military command has reported no move, however, by Argentine troops to launch an attack on the British perimeter.

According to official accounts, this battle strategy is quickly exhausting the ability of the British to maintain their beachhead on the island. Since Friday, Argentina has reported sinking two British ships and damaging 11 others, many of them seriously. The British, Frugoli argued today, "don't have a way to reprovision their troops" at San Carlos.

Meanwhile, ignoring British reports that more than 30 Argentine aircraft have been shot down in the air attacks, the military command, in two late-night communiques, had by today acknowledged losing only seven planes and three helicopters since last Friday. In all, Argentina's air forces have had only 15 planes put out of action since the beginning of the conflict with Britain, according to these reports.

There were several indications here today, however, that Argentina would seek military help from other nations if its air power and ground troops could not drive the British off the islands.

Frugoli said today that "the risk is being run not only of a worsening of the conflict but of an internationalization." The defense minister also did not deny a report in the progovernment newspaper La Nacion that Argentina could seek military aid from the Soviet Bloc. "It is up to Argentina to leave open or not important Soviet military supply routes," the newspaper quoted a "high military leader" as saying.

The newspaper's account also carried one of the first suggestions that Argentina may be suffering large military losses, quoting the military official as saying that eventually, "We will be left without equipment, unless we accept that which is offered to us, regardless of where it comes from."

Argentine officials have hinted in the past that they could turn to the East for help if left isolated against a British attack. High officials have later denied these suggestions, which have been described as part of an effort to prevent support for Britain by the United States and Western Europe.

Frugoli today strongly called on the United States to use its influence to end the fighting, saying that "the United States is the most important country in the West and consequently cannot elude the responsibility that comes with that."

This call for U.S. support, a common theme of the Argentine government during the past several weeks, was also coupled with an increased tone of bitterness in some government statements.

Foreign Ministry Cabinet Chief Gustavo Figueroa said that in view of continued U.S. support for Britain, he was beginning to believe reports that the United States wanted to help defeat Argentina so it could establish its own military base on the Falklands. He also charged that U.S. newspapers had been "absolutely arbitrary" in their coverage of the conflict and had been ignoring Argentina's account of British losses in the fighting.

Foreign Minister Costa Mendez said at the United Nations in New York that Argentina has decided to remove some or all of its personnel from the Inter-American Defense Board in Washington, according to reports reaching here.

Government officials also reacted sharply today to the European Community's decision to extend sanctions against Argentina indefinitely. A statement issued on behalf of Economy Minister Roberto Alemann said the new measure would force Argentina to continue blocking imports from the community.

In the past, Alemann and private economists here have said that the community trade blockade, though amounting to some 20 percent of total Argentine sales abroad last year, has not had a significant effect on the economy here. However, analysts have said that a continuation of the blockade over several months might hurt such Argentine exports as meat, fruit, and some cereal products.

Some economists here have also warned that the community sanctions could eventually risk Argentina's ability to make payments on its $34 billion foreign debt, much of which is held by European banks.