The Argentine government announced tonight that the cruiser General Belgrano, the second largest vessel in the country's Navy, had sunk in the South Atlantic after being torpedoed by a British submarine last night. More than 900 crewmen were reported missing.

It was the greatest military loss suffered by Argentina in a week of military confrontations with Britain over possession of the Falkland Islands.

The Foreign Ministry announced the cruiser carried a crew of 1,042, and the military command said that five hours after the attack 123 survivors had been rescued from the wind-whipped, icy ocean. An Argentine plane had spotted life rafts in the area, and vessels were rushing to the scene to search for additional survivors.

"At 5 p.m. 4 p.m. EDT May 2, the Argentine Navy cruiser General Belgrano was attacked and sunk," a Foreign Ministry communique said.

It termed the attack "a treacherous act of armed aggression" in open violation of the United Nations Charter.

Military officials made no comment on claims by Britain that its helicopters sank one Argentine patrol boat and damaged another early this morning, hours after the attack on the cruiser.

The Belgrano, according to Argentine officials, was attacked off the tip of Tierra del Fuego, south of the Falklands and just outside the 200-mile war zone Britain has proclaimed around the islands.

The 13,645-ton, 600-foot-long vessel was formerly a U.S. warship, the USS Phoenix. It was built in 1938 and survived the Japanese attack on Pearl Harbor. The United States sold it to Argentina in 1951. It had 15 six-inch guns and two launchers for surface-to-air missiles but was not considered a key element in the military balance because of its age.

The British Defense Ministry had said early Monday that the Belgrano suffered "severe damage" after being hit by two torpedoes. After Argentina announced the sinking, the Ministry said it had no further word on the cruiser, but that the British task force was taking emergency steps to rescue any crewmen.

Ministry spokesman Ian McDonald acknowledged that the Argentine warship was outside the 200-mile zone but he said it "presented a significant threat to our ships in the area of the Falkland Islands," Washington Post correspondent Jay Ross reported from London.

McDonald noted, however, that in addition to the 200-mile zone, Britain has proclaimed a "bubble" zone around vessels of the task force, including submarines, and has warned that any Argentine warships in the vicinity of the ships "would encounter appropriate response."

Britain said its helicopters attacked the two Argentine patrol boats north of East Falkland island, 90 miles within the 200-mile zone, after the boats fired on a British Sea King reconnaissance helicopter. Lynx helicopters from task force vessels then attacked the boats with missiles, McDonald said, sinking one and damaging the other with no loss to the British.

Britain also requisitioned the Queen Elizabeth 2 luxury liner to carry an infantry brigade of about 3,000 men to the South Atlantic, to raise to about 8,000 the number of British combat troops committed to the recovery of the Falklands.

Despite indications of continued naval losses in the South Atlantic, Argentina's military government continued to maintain today that it holds the strategic advantage over Britain's task force. Support for the military junta's determination to continue fighting for the Falkland Islands showed no signs of slackening.

While reiterating their intent to continue resisting British attacks, Argentine officials gave no indication today that this country would try to take the initiative with an air or naval attack on British forces.

Military officials stressed their interpretation that Britain was rudely surprised in three attempts to land troops on the Falkland Islands Saturday. Argentine forces, dug into the territory, have demonstrated to the British that they could not recapture the island without a costly battle, the officials said.

Meanwhile, there was an indication of new Argentine diplomatic movement as President Leopoldo Galtieri dispatched two of his top aides to meet with Peruvian President Fernando Belaunde Terry.

Argentina said early today that it had rejected a new U.S. plan transmitted through Belaunde's government for a solution to the confrontation with Britain, which began April 2 when Argentina seized the British-held islands after a long dispute and unsuccessful negotiations over their possession.

The plan, Argentina said, was substantially similar to a previous U.S. formula, presented by Secretary of State Alexander M. Haig Jr. when Argentine Foreign Minister Nicanor Costa Mendez was in Washington last week. Argentina's rejection of that plan led to a shift by the United States to open support for Britain in the crisis and the announcement of sanctions against Argentina.

No official explanation was offered today for the trip to Lima by Gen. Hector Iglesias, general secretary of the presidency, and Rear Adm. Benito Moya, the chief of the presidential military office.

Officials indicated that the two carried a detailed response to the plan transmitted to Argentina by Peru on Saturday, and they said counterproposals by Argentina were possible.

Argentine military officials have given no indication in the past several days that they intend to change what has been a strategy of resisting British attacks while holding to an entrenched defensive position on the occupied islands.

Argentine officials, including Galtieri, have carefully stressed in recent days that Argentina will respond to British attacks without suggesting that Argentine forces would seek out the British.

Argentine analysts familiar with the military situation say the ruling junta will likely continue the strategy for a variety of reasons.

Even before the loss of its second largest warship and one of its four submarines, Argentina's Navy clearly was no match for the British fleet, analysts here say. The Belgrano was not far from Argentine territory on Tierra del Fuego when it was attacked and analysts here say the Argentine fleet is likely to remain near the coast to guard crucial naval and air bases in southern ports. Argentina's largest warship is the aircraft carrier 25th of May.

Only the Argentine Air Force might be capable of launching a successful attack on the British fleet. In the past, military analysts here have suggested that an all-out attack by Argentina's more than 100 high-performance fighter-bombers might try to sink or severely damage one of Britain's two aircraft carriers and thus gravely hurt British chances of maintaining a blockade around the islands.

But an Argentine air attack would be difficult because the main British fleet, believed to be at least 100 miles east of the Falklands, is almost 500 miles from Argentina's nearest coastal air bases at Rio Gallegos and Rio Grande, analysts said.

Argentine A4 Skyhawks, Mirages and Canberras heavily loaded with bombs would not have much fuel for air battles with Britain's carrier-based fighters at such a distance. Analysts here believe Argentina has stationed few or none of its sophisticated jets on the Falklands themselves because of concern they would be destroyed on the ground by British bombing or shelling.

Galtieri and the two other members of the military junta do not appear to face strong political pressure for quick military action.

As long as Argentine forces do not suffer a decisive military setback or yield control of the islands to Britain, the junta's position remains relatively safe, observers here say.

But political analysts cautioned tonight that if the sinking of the cruiser results in heavy casualties, it could mean a setback that disrupts the junta's control.

Galtieri told Argentines in a televised address Saturday to expect "a high cost" in materiel and lives in the fight for the Falklands. That conclusion has not been questioned by political and labor leaders here, many of whom have continued to call for a strong Argentine stand.

The leader of the Radical Party, Carlos Contin, told Interior Minister Alfredo Saint Jean yesterday his party offered "complete solidarity" for the military's operations against the British. He added that the party was willing to "lend its unlimited and unconditional support in the form the government considers necessary to consolidate" Argentina's control of the islands, according to an Interior Ministry statement.

The Peronist Party, believed to be the most powerful civilian force in Argentina, also called after the weekend's fighting for the "creation of a monolithic national unity, which will be, undoubtedly, a triumphal procession against outrage and irresponsible aggression."

Both Contin and the Peronists said they continue to hope for a successful diplomatic solution to the crisis, and one alternative peace plan has been offered by a leading civilian politician here. However, government officials today expressed few hopes for a quick diplomatic breakthrough or a successful mediation by Peru.

The government appeared to be pursuing few other diplomatic options outside of the trip by Galtieri's aides to Lima. Officials said they remained open to action by the United Nations or the Organization of American States but they said Argentina was not considering changing its negotiating stance of insisting on guarantees of sovereignty over the Falklands and the associated, outlying South Georgia and South Sandwich Islands.

The last plan offered to Argentina by the United States proposed a withdrawal of the Argentine troops that have occupied the Falklands since April 2 along with a recall of the British naval forces. The plan called for a temporary tripartite government for the islands including Argentine, British, and U.S. representatives and long-term negotiations over the islands' future, according to government statements here.

Argentina has called this and similar formulas unacceptable because they do not guarantee Argentine sovereignty over the Falklands within a fixed period.

One Foreign Ministry source said today that while Argentina was expecting action in the United Nations, it was unlikely to pursue new initiatives by countries that are signatories to the Inter-American Treaty of Reciprocal Defense, known as the Rio Treaty.

"We got the political support that we wanted already," this official said. "Anything else--such as sanctions--would be impossible to enforce with the United States against us."

Argentina won support from treaty signatories in a resolution passed last week recognizing Argentina's claim over the Falklands and officials have pointed out that the special treaty consultative session remains open for further action.

Washington Post correspondent Jay Ross reported from London:

The sinking of the General Belgrano outside the 200-mile British blockade zone, particularly if accompanied by heavy loss of life, could become an issue in this country where "rules of engagement" take on great importance.

Although Britain has proclaimed a "bubble" around the task force and warned Argentine vessels against intruding, it could be argued that since the Belgrano was attacked by a submerged submarine, the Argentine skipper did not know he was near the task force.

Reporters pressed British officials on the issue. Defense Ministry spokesman McDonald turned aside a question whether this meant that British ships would be within their rights to attack Argentine vessels outside the zone and close to the mainland. He would only say the "bubble" is not static.

Defense Minister John Nott was asked at a press conference whether the attack on the Belgrano was consistent with what he had earlier called Britain's policy of using the minimum force necessary to gain an Argentine withdrawal.

Nott and McDonald simply repeated that the Belgrano "presented a threat" to the task force.

An aide to Prime Minister Margaret Thatcher said she was informed about the torpedoing of the cruiser at about 1 a.m. London time and her consent was not sought in advance.

The attack on the Argentine patrol boats north of the Falklands were the British Navy's first combat use of its Skua missiles.

A British correspondent on the task force interviewed one of the helicopter pilots, who said that after the first patrol boat, believed to be a converted tug, was sunk a helicopter was sent to look for survivors but was fired on by the second patrol boat, believed to be a corvette with four-inch guns. The British helicopter then fired on the ship, which caught fire.

The helicopters returned safely, the spokesman said, and British aircraft later dropped life rafts near the damaged vessel. There was no information on survivors from either ship, which may have carried as many as 28 crewmen each.

The Thatcher aide said that "for the moment at least" any further action in the area would depend on "whether the Argentines want to put themselves in a menacing position," an implication that Britain plans no major offensive action. He refused, however, to call it a pause in the action and said there was no link to the diplomatic efforts of Foreign Secretary Francis Pym.

Pym returned from the United States tonight where he saw Secretary of State Alexander M. Haig Jr. and U.N. Secretary General Javier Perez de Cuellar. He is to brief Thatcher and the Cabinet tomorrow morning.

Thatcher will also decide tomorrow whether to meet with two of the three opposition leaders. Liberal Party leader David Steel and David Owen, foreign policy spokesman for the new Social Democratic Party, are seeking a meeting with her on the Falklands crisis.

Thatcher first offered to hold the meeting but it has now been thrown in doubt because of the refusal of the main opposition leader, Labor's Michael Foot, to attend.