Argentine warplanes staged a "mass air attack" yesterday afternoon on British ships guarding the Falkland Islands beachhead and left at least one vessel "badly damaged and in difficulty," British Defense Secretary John Nott announced.

British defense sources said the damaged ship was a destroyer, which carries a crew of about 280. Rescue operations were under way, a military spokesman said, but there was no information on casualties.

An Argentine military communique said the air attack had seriously damaged a British warship, sending up "a great cloud of smoke." It said two British frigates suffered lesser damage, a medium-sized troop transport was probably hit, and one Argentine plane was lost.

Nott said in a television interview in London that the assault was "quite a significant and major attack on our ships," adding: "The Argentine pilots are showing great bravery."

The air strike climaxed the fourth day of fighting since British troops landed on San Carlos Bay on the western coast of East Falkland island and established their beachhead.

British ships earlier in the day shot down three Argentine Skyhawk jets that were carrying out smaller raids, the Defense Ministry said. British defense sources and correspondents said British troops had begun "aggressively patrolling" East Falkland in combat helicopters and had fired on suspected Argentine troop positions outside the expanding, British-held perimeter.

In Washington, Secretary of State Alexander M. Haig Jr. told President Reagan that Britain was in a position to quickly defeat Argentine forces in the Falklands. Administration officials are understood to believe that the British are likely to recapture the islands within 10 days.

Diplomatic maneuvering continued but offered little hope for a truce. Argentine Foreign Minister Nicanor Costa Mendez called on the U.N. Security Council to impose a cease-fire and order a resumption of negotiations under the U.N. secretary general.

But British Prime Minister Margaret Thatcher said in some of her toughest language to date that an Argentine withdrawal would have to precede any halt in combat.

"We totally reject a cease-fire ," Thatcher told Parliament. "That would leave the whole paraphernalia of tyranny in place." She said Britain would veto a cease-fire resolution in the Security Council if necessary.

The Vatican, ending weeks of uncertainty caused by the Falklands conflict, announced that Pope John Paul II will go ahead with his six-day trip to Britain starting Friday.

The pontiff also sent a letter to Argentine President Leopoldo Galtieri offering to come directly to Argentina from Britain, the Vatican's ambassador to Argentina reported, in an apparent effort to demonstrate papal evenhandedness in the conflict.

Argentine government officials indicated that Buenos Aires would seek to establish a diplomatic front of developing nations to pressure Britain to end its military offensive. A government statement condemned Monday's decision by eight European Community countries to extend economic sanctions against Argentina indefinitely, calling the move "an arbitrary and illegal form of commercial coercion with the object of bending the will of a developing country."

British Harrier jets attacked the Falklands' main airfield at Stanley, and the Defense Ministry in London hinted that some of the planes may have been launched for the first time from the British beachhead instead of from aircraft carriers. Argentina said Stanley's defenders had shot down three Harriers, but Britain said all its planes returned safely.

The British Defense Ministry has been silent in its public statements about the activities of the 5,000 troops that it claims to have on East Falkland. Military analysts suspect the reticence may indicate that a major operation is under way, with speculation focusing on possible raids on the Goose Green and Stanley airfields.

British correspondents reported that British troops on patrol outside the beachhead have captured a few Argentine soldiers.

Argentina said in its military communique that its troops, supported by small, land-based aircraft, were "carrying out missions" against the British beachhead yesterday evening "to gain control over the enemy and limit his movement."

A British Broadcasting Corp. correspondent said that British forces had spotted Argentine troops approaching the vicinity of the beachhead on Monday night for the first time.

A small Argentine unit, believed to be coming from the Darwin-Goose Green area south of San Carlos, was spotted by a British observation post on the perimeter of British-controlled territory, the BBC's Jeremy Hands reported. The British fired mortars and artillery in the Argentines' direction for much of the night, but there was no answering fire, he said.

The Argentine air attacks on the British fleet were the fourth in five days on the ships in and around Falkland Sound between the two main Falkland islands. The Argentines are trying to take advantage of their numerical superiority in aircraft against the British forces, but Britain claims it has knocked out a total of 70 Argentine aircraft in four weeks of fighting in the South Atlantic .

British ships hit two of the three Skyhawks downed yesterday with missiles, the Defense Ministry said. It did not say how the third was shot down.

The loss of a destroyer would be the worst British military setback since the sinking of the destroyer Sheffield three weeks ago. In addition to the Sheffield, the British have acknowledged that the Argentines have sunk two frigates, the Antelope and the Ardent, both since Friday's landing.

The Defense Ministry admitted yesterday that Argentine warplanes had damaged two support ships in Falkland Sound on Monday but said both were "being made good."

Britain denied Argentine reports that the luxury liner Canberra, now used as a troop transport, had been damaged.

"I deny the latest wild report from Argentina claiming the Canberra has come under attack," Secretary Nott told the House of Commons.

Before yesterday's action, defense sources said additional British warships had been moved to the bay and that the task force overall had more ships, with new arrivals, than ever before.

BBC correspondent Brian Hanrahan on one of the British warships still in San Carlos Bay just off the British land perimeter reported that the bulk of heavy equipment and supplies has now been taken ashore. Even a field hospital has been built there, he said, where an Argentine pilot picked up from the bay is now being treated after he parachuted from his Skyhawk as it was shot down.

Hanrahan said extra guns have been mounted on the ships in the bay and that the proportion of Argentine planes hit by fire from the ships has been steadily rising.

Additional Blowpipe shoulder-held antiaircraft missiles also have been brought in for ground troops to use to defend themselves from air attack.

In Chile, three British crewmen were found in good condition a week after their helicopter apparently crash-landed in the country, the Chilean Foreign Ministry reported.It said the three will be handed over to the British Embassy.

The three were found near Punta Arenas in southern Chile and had thought they were in Argentina, radio reports said.

The Chilean government, which had discovered the burned-out helicopter earlier, had protested the incursion in a diplomatic note to Britain. Later, however, Chile accepted the British government's explanation that the helicopter had gone astray in bad weather during a reconnaissance mission.