British commandos, making their first reported landing on the Falkland Islands, staged a hit-and-run raid last night on a small, outlying airstrip used by Argentine forces and destroyed 11 planes on the ground and blew up a large ammunition dump, the Ministry of Defense announced.

British defense sources said the Argentines were believed to have suffered casualties. A ministry spokesman said all the British troops returned to their ships offshore at dawn today "with only two minor casualties."

The Argentine government said a "surface unit" had attacked the airstrip and damaged three planes. It did not refer to a commando raid and gave no reports on casualties.

The British said the commandos assaulted the airstrip on Pebble Island after landing in darkness from helicopters as British warships bombarded Argentine positions. The island is just off the northern coast of West Falkland Island, about 80 miles northwest of Stanley, the capital of the disputed territory seized by Argentina April 2 after 149 years of British administration.

Earlier yesterday, the Stanley airport and Argentine military installations around it were again bombed by Sea Harrier jets from an aircraft carrier in the British naval task force deployed around the islands.

The Defense Ministry here refused to say how many British troops, believed to be Marines, participated in the raid on the 19-mile-long island, whose 600-yard airstrip is the closest to Argentina of many such strips on the Falklands.

"I must emphasize that this was a raid and not an invasion of the Falkland Islands," spokesman John Wright said. "It was a limited military action designed and executed as part of the task force's continuing role in enforcing the total exclusion zone." Britain has threatened to attack any Argentine plane or ship found on or around the Falklands.

The raid also was part of a step-by-step military escalation by British forces, which government sources said will soon lead to large-scale landings unless there is an unexpected breakthrough in negotiations for a settlement with Argentina.

Such hit-and-run raids, of which more are expected, were described here as necessary to reduce the ability of the Argentine occupation forces to resist an invasion, to test their fighting ability and to discover their defensive plans, perhaps by taking and interrogating prisoners. The Defense Ministry provided no information today about Argentine resistance to last night's raid.

The ministry spokesman said six Pucara planes, designed for attacks on ground forces, were among the 11 Argentine aircraft the British claimed to have destroyed on Pebble Island. The low-flying twin-turboprop Pucaras, each armed with two 20-mm cannons, four 7.62-mm machine guns, rockets and bombs, have been used effectively against domestic guerrillas in Argentina and could pose a danger to British invasion forces.

Other Pucaras have been damaged in repeated British aerial bombing and naval bombardment of the air fields at Stanley and Goose Green on East Falkland Island, according to the British.

Also among the destroyed aircraft was a Skyvan, normally used for carrying freight, the spokesman added. He did not identify the rest of the planes.

The British forces apparently detected the aircraft on Pebble Island through reconnaissance by aircraft or by specially trained commandos who may have landed on the Falklands some time ago. The airplanes were probably placed there to guard against an invasion of West Falkland from the north.

Informed British sources said Prime Minister Margaret Thatcher's government has not yet decided to invade the Falklands. But they added that daily decisions made by Thatcher's "war cabinet" of senior ministers to steadily increase military pressure could be leading them toward an invasion decision within a few days.

Britain's military commanders also reportedly are recommending such a decision now that the assault force of 4,000 to 5,000 men has been assembled on British ships off the Falklands, where they could lose their fighting edge and are targets for Argentine air or submarine attacks.

Britain's U.N. ambassador, Anthony Parsons, and its U.S. ambassador, Nicholas Henderson, returned here and met with Foreign Secretary Francis Pym and other senior officials to start shaping Britain's final position in the Falklands talks being conducted by U.N. Secretary General Javier Perez de Cuellar.

Officials here said they remain pessimistic about the U.N. negotiations. Parsons told reporters here that common ground has been achieved through Perez de Cuellar on some points but, "It is quite impossible to speculate how they are going to come out."

"We are getting to the heart of a number of difficult problems," he said, adding that there had been "a change at least on the face of it in the Argentine position in prejudging the negotiations on sovereignty. The question is whether what we have seen in New York will stick. We will need to see the whole package."

One government source emphasized that Thatcher does not face "an apocalyptic choice between war and peace" this weekend because Britain has simultaneously increased military pressure and continued negotiations since the Argentine invasion.

Parsons and Henderson will meet with Thatcher when she returns from Scotland Sunday and will be advising her, according to a source, "on how best to carry forward our strategy, both military and diplomatic," in the crucial days ahead.

It is quite possible, this source said, that British troops could land on the Falklands in large numbers while British diplomats continue seeking a negotiated settlement either at the United Nations or through another diplomatic channel. British officials refused to comment on the recent trip to Argentina by roving U.S. Ambassador Vernon Walters or on what message Henderson might be bringing Thatcher from the Reagan administration.