Britain's invasion of the Falkland Islands yesterday was the largest single military action yet in its sporadic month-long fighting with Argentina in the South Atlantic. But few casualty figures were made available by the two sides.

The only British casualty reports were the loss of 21 men when a Sea King helicopter ferrying them between ships crashed into the sea, and three other deaths reported by the Defense Ministry early this morning. One body was recovered from the helicopter crash and 20 others were missing and presumed dead. The British Defense Ministry also reported serious damage to two warships and unspecified damage to three others and the loss of one Sea Harrier jet and two small helicopters. Argentina said it shot down three Harriers and two helicopters.

The deaths brought to 48 Britain's total reported dead in Falklands military operations to date. Included are 20 seamen killed in a missile attack on the destroyer Sheffield, three pilots and a seaman who died accidentally.

Argentina said last night that it "put out of action" eight British frigates, two of which "could have sunk."

Before yesterday's fighting, the British had reported losing three Sea Harriers and six other helicopters. Besides the Sheffield, sunk a week after it was attacked, London had reported "minor damage" to several frigates.

The British said they destroyed nine Argentine Mirage warplanes, five Skyhawks and two Pucaras in yesterday's fighting. Argentina conceded that it lost three planes and three helicopters. There were no reports of the number of Argentine casualties.

So far in the Falklands war, Argentina's Army and Air Force has reported 53 dead and 62 wounded. The Argentine Navy said it has suffered 20 deaths and 301 seamen missing from the sinking of the cruiser General Belgrano. Until yesterday, other damage included the crippling of the Argentine submarine Santa Fe, and the loss of two to three Skyhawks, seven Pucaras, two Mirages, one Canberra, two helicopters and six patrol and supply vessels.