A British submarine torpedoed and is believed to have "severely damaged" an Argentine warship just outside Britain's 200-mile blockade zone around the Falkland Islands yesterday afternoon, the Defense Ministry announced here early this morning.

The Argentine cruiser, the General Belgrano, had "posed a significant threat to the British task force" enforcing the blockade, a Defense Ministry statement said, and was attacked at about 3 p.m. EDT by a submarine guarding the task force perimeter. The ministry said the British submarine was not damaged during the attack and has resumed patrolling.

The Argentine joint chiefs of staff issued a communique confirming that the General Belgrano had been attacked and damaged by a torpedo, Washington Post correspondent Jackson Diehl reported from Buenos Aires.

The statement said that boats had been sent to the area to help the cruiser "in case it is necessary" but gave no details on the extent of the damage or whether any casualties were suffered. It said the attack took place southeast of Los Estados Island, which is east of Tierra del Fuego at the southern tip of Argentina and southwest of the Falklands.

"It is worth pointing out that the attack took place . . . outside of the denominated zone of exclusion," the communique said.

The General Belgrano, Argentina's only cruiser, was launched in 1938 by the United States. Originally commissioned as the USS Phoenix, it survived the Japanese attack on Pearl Harbor and was later sold to Argentina. It has been outfitted with Seacat surface-to-air missiles and also can carry two helicopters.

Although the cruiser was just outside the declared blockade zone, the British statement said, it was liable to attack under the warning Britain issued on April 23 that "any approach by warships or aircraft which could amount to a threat on British forces would encounter an appropriate response."

The ministry said the Argentine cruiser posed such a threat, and the attack on it was "fully in accordance with instructions given to the task force commander based on the right to self-defense under Article 51 of the United Nations Charter."

The attack, about which the ministry said there now were no more available details, was the only combat reported Sunday by either Britain or Argentina after 20 hours of bombing raids, aerial dogfights and naval bombardment in the South Atlantic conflict.

British government sources and military analysts said Saturday's attack had succeeded in cutting off Argentine forces on the islands from resupply by air. They said the next step would be new military pressure in the form of commando raids, landings by British forces, further aerial attacks and naval bombardment, all designed to force Argentina to withdraw its 5,000 to 10,000 troops from the Falklands.

The British Defense Ministry said yesterday that Argentina lost at least three warplanes and suffered heavy damage to two airfields on the Falklands, while British forces escaped without serious casualties. A ministry spokesman said Argentine antiaircraft gunners around the Port Stanley airfield may even have shot down one of their own supersonic Mirage jet fighters. British journalists aboard the aircraft carrier Hermes reported that four Argentine planes were shot down.

The Argentine military command gave a much different account, saying its forces had shot down at least five British jets and probably destroyed six others, and had severely damaged a British frigate, Diehl reported. Other reports citing military sources also claimed that the Argentines had badly damaged the Hermes, flagship of the British naval task force.

A communique from the Argentine joint chiefs also said British forces twice tried to land on the Falklands using helicopters but were repulsed by Argentine troops. It reiterated earlier claims that the British attack did not damage the Port Stanley airfield.

Argentina conceded it had lost two Dagger warplanes, an Israeli-made version of the French Mirage, but claimed it had won a major victory. "We Repelled Them With Blood and Fire" read the headline of one Buenos Aires newspaper.

While Argentina had attributed yesterday's initial lull in the fighting to the strong counterattack that it said had crippled Britain's naval task force, British officials said the lull was designed to give Argentina's military government an opportunity to "stew over" and "take stock of" the damage inflicted Saturday.

"They know they are cut off now," a senior British government source said. "Don't expect a massive invasion soon, but we shall keep turning the screw. We shall keep reminding them of what's coming and that anything can happen at any time."

Government sources here said Prime Minister Margaret Thatcher also was assessing the political impact of the escalation of the conflict and the possibilities for new diplomatic moves. Thatcher briefed leaders from the opposition Social Democratic and Liberal parties yesterday on the British attack, although Labor Party leader Michael Foot declined to attend.

The Defense Ministry here denied Argentina's claims of British warplane casualties, saying no British planes or helicopters were lost and that a British ship hit during Saturday night's Argentine bombing raid on the task force suffered "only superficial splinter damage" from shrapnel.

The only reported British casualty--Britain's first in the conflict, including last weekend's capture of South Georgia Island--was a seaman on the damaged ship. The seaman, Ian Britnell, 20, from Teignmouth in southwestern England, was reported in "serious but not critical" condition after being hit in the chest by shrapnel. Correspondents with the fleet said last night that Britnell was "safe and comfortable" after surgery aboard ship.

What Conservative member of Parliament Alan Clark yesterday called the "miraculous" avoidance of British combat deaths so far is an important point of credibility for the Thatcher government. Opinion polls consistently have indicated that the current overwhelming support for Thatcher's handling of the crisis could quickly evaporate with significant British casualties.

The Defense Ministry refused comment on other reports from Buenos Aires, including the claim that British troops had made abortive attempts to land on the islands. A week ago, naval bombardment similar to that trained on the Port Stanley airfield was used to cover helicopter landings by British troops in the recapture of South Georgia.

A senior government source said he would expect Britain's next military moves to continue to be "surgical" operations to minimize casualties. "I can't stress enough how reluctant we are in this conflict," he said, responding to questions about whether Thatcher was moving inevitably toward a full-scale assault on Port Stanley.

The Defense Ministry here also denied an Argentine report that the Hermes was disabled by Saturday's counterattack. "We categorically deny that there have been further attacks by their forces since Saturday night or that the Hermes or any other British ship has been damaged," a spokesman said.

"Both sides have a credibility problem," a Thatcher aide acknowledged yesterday, referring to the claims and counterclaims traded by Britain and Argentina. "I can only say that our ministers have to answer for everything daily in Parliament in a way that no one in the junta does. I don't think this government could get away with lying."

The British government has provided only sketchy details of the combat known to have taken place Saturday. But from that information, Argentine communiques and accounts by British correspondents reporting under military censorship from task force ships, it is possible to reconstruct much of the fast-moving battle.

The raid began before dawn when a single, delta-winged Vulcan bomber flying 3,200 miles from Ascension Island dropped 21 1,000-pound bombs on the 4,000-foot runway at Port Stanley airport. The high-explosive, delayed-detonation bombs could have blown craters 15 feet deep in the runway, according to defense officials here.

The Vulcan was guided to its target by radar aboard the Hermes, according to British correspondents on the carrier. After maintaining radio silence during the bombing run, which apparently took the Argentine occupation forces by surprise, the pilot radioed to the carrier a code word for a successful mission, the correspondents reported.

At dawn, Harriers from the Hermes launched a second raid on the Port Stanley airport and also attacked a grass airstrip at Goose Green, 55 miles west. The Harriers each dropped three, 1,000-pound bombs and strafed ammunition and fuel dumps with two-inch rockets.

The now alerted Argentine defenders answered the attack with heavy antiaircraft fire. A British Broadcasting Corp. correspondent aboard the Hermes said a bullet hole in the tail of one of the returning Harriers was was the only damage suffered.

Defense Ministry spokesman Ian MacDonald told a press conference yesterday that the raids resulted in "a severely damaged runway at Port Stanley airfield and considerable damage to military installations and stores." He said the Goose Green airstrip, where most of the Argentine warplanes on the islands apparently were being kept, "was damaged, together with a number of Argentine military aircraft." The damage was confirmed by reconnaissance photographs, according to correspondents on the Hermes.

The Hermes and its escort ships had moved to within 90 miles of the eastern coast of East Falkland so that the short-range, vertical-takeoff Harriers could launch the raids. This put the task force well within the 200-mile blockade zone that both Britain and Argentina have declared around the Falklands and just within the range of Argentina's mainland-based Mirage and Dagger fighter-bombers.

By late Saturday afternoon, three British warships had advanced to within 12 miles of Port Stanley. At 6 p.m (5 p.m. EDT) they began shelling the airfield "to reinforce the effects of the bombing and to deter repair work," defense spokesman MacDonald said.

"While this bombardment was taking place," MacDonald said, "Argentine fighter and bomber aircraft attacked the ships." Harriers that had been patrolling overhead to guard the task force against such an attack engaged the Argentine Mirages in what one British correspondent aboard the Hermes called "a fierce dogfight."

Firing heat-seeking Sidewinder missiles, the Harriers shot down one Mirage, Macdonald said, and another Mirage "is believed to have been shot down by Argentinian gunfire from around the Port Stanley airport."

Later Saturday night, Argentine pilots flying aging British-made Canberra bombers made a bombing raid on the task force ships that MacDonald described as "quite serious in intensity" but "ineffective."

MacDonald said one of the Canberras was shot down by Harriers defending the ships and another was believed to be "severely damaged." Defense Ministry sources said no surviving crewmen from the crashed planes were found by Harriers searching for them.

Describing Saturday night's raids on the task force from the Hermes, BBC correspondent Brian Hanrahan reported that the Argentine warplanes "dive-bombed the ships, but missed them. The ships were then strafed and two of them suffered what was described as superficial injuries."

Hanrahan said the task force commander, Rear Adm. Sandy Woodward, had relaxed censorship and allowed reporting of the combat to refute Argentine claims that the Hermes and other ships had been disabled. "The Hermes has not even been attacked," Hanrahan reported. He said routine patrols over and around the task force by Harriers and Sea King combat helicopters continued yesterday without incident.

In an interview with Britain's Press Association, Harrier pilot Bertie Penfold, described how he shot down an oncoming Argentine Mirage fighter. "I locked a Sidewinder missile onto his jet wake and, after three or four seconds, the missile hit."