British troops stormed the Falkland Islands early yesterday morning under cover of heavy air and naval bombardment as the long-awaited assault to reclaim the South Atlantic colony from Argentina began.

About 2,500 Royal Marine commandos and paratroopers established what the British Defense Ministry called "a firm bridgehead" in four separate landings around the port of San Carlos in the Falkland Sound dividing the two main islands. Britain said the landings were unopposed.

Argentine warplanes counterattacked later, damaging five British warships in or near the Falkland Sound, according to the British. Argentina's military command said its forces "put out of action" eight British frigates. Of those, four suffered "great" damage and two "could have sunk," the military command said last night. It insisted earlier that the British had failed to secure a beachhead on the islands.

By Argentina's official count, Britain lost three Sea Harrier planes and two helicopters.

British Defense Secretary John Nott, in confirming that Argentine planes had hit five British warships, said two of them were damaged seriously. Officials confirmed 21 British dead in a predawn helicopter crash that they said was an accident. Early this morning, the Defense Ministry added that there were three dead, two missing and 27 wounded in yesterday's fighting.

The Defense Ministry said that British Sea Harriers had destroyed 16 Argentine warplanes and damaged another and that two Argentine helicopters were destroyed on the ground. Britain said it lost two helicopters and reported one Sea Harrier missing.

In Washington, the White House released a statement reiterating its pledge to "meet our commitments to Great Britain" but stressed that "there will be no involvement whatsoever of U.S. military personnel in the conflict in the South Atlantic."

At the United Nations, Secretary General Javier Perez de Cuellar suggested to an emergency meeting of the Security Council that both Britain and Argentina were to blame for the new outbreak of fighting following the collapse of his effort to achieve a diplomatic settlement of the dispute. The council took no action yesterday. Britain is expected to veto any call for an unconditional cease-fire.

As of last night, both sides said fighting was continuing but otherwise disagreed sharply on the outcome of the day's events. Britain said its forces were in firm control, and sources said more men and equipment would be landed during the night on the islands, which Argentine forces seized April 2.

"Seven weeks after the Argentine aggression, British forces are tonight firmly established back on the Falkland Islands," Nott said.

But Argentina described the British naval task force as trapped in the narrow Falkland Sound and British troops as pinned down on shore. It said its land-based forces were engaged in repelling the invaders.

In a television interview last night, Argentine Defense Minister Amadeo Frugoli said the British had suffered heavy losses of troops and materiel. He said Argentine troops were in a "clearly favorable" position in driving back the landing attempts.

Yesterday's British landing was preceded by a series of predawn commando raids on military targets including fuel and ammunition dumps. While transferring troops between ships in preparation for the raids, Britain reported, a Sea King helicopter with 30 men aboard accidentally crashed into the wind-tossed South Atlantic. Nine men were reported rescued, one body was recovered and 20 others were missing and presumed dead. It was the single largest loss of life Britain has reported in a month of sporadic hostilities.

Nott said the raids resulted in an unspecified number of Argentine casualties and prisoners. He said some of the commandos had remained ashore and that some of the operations were continuing.

Yesterday's fighting began with commando raids and air and naval bombardments of the Falklands capital, Stanley; other areas in East Falkland, and on the Fox Bay area on West Falkland.

While the Argentine defenders were diverted by these attacks, Royal Marines and paratroopers operating in bright moonlight landed on four largely undefended beaches near San Carlos, British sources said. The first landing craft reportedly hit the beach at 6:30 a.m. EDT.

Sources said that light tanks, mobile missile launchers, artillery, jeeps and mortars were all brought ashore. There were also several hundred engineers and support troops who were assigned to prepare at least one landing strip for Sea Harriers during darkness last night.

About three hours after the landing, Argentina counterattacked, first with Argentine-built Pucaras, then with French-built Mirages and American-made Skyhawks. The government news agency Telam quoted the military administration in the islands as saying there were seven air attacks in all, involving up to 16 planes each. There were reports from Buenos Aires that the Pucaras resumed attacks on the bridgehead after dark.

Britain reported that Argentina lost nine Mirage aircraft, five Skyhawks and two Pucaras and that one other Mirage was damaged. Argentina conceded only that it lost six planes and three helicopters. It gave no reports of Argentine casualties.

A high-ranking U.S. government source said the sharply escalated fighting marked a dangerous and unpredictable phase in which one side probably will have to gain a decisive military advantage before there is a chance to negotiate an end to the hostilities, he said.

Another administration source said the United States is sending aerial tankers to NATO to replace British aircraft withdrawn from the alliance for use in supporting the Falklands fleet.