Argentina's military command claimed tonight that its jets caused heavy damage to an aircraft carrier in the British task force off the Falkland Islands today as its troops appeared to be reorganizing its defense of the islands in hopes of halting the offensive of British ground forces with a last, determined stand.

The British Defense Ministry earlier denied Argentine news reports that the carrier HMS Invincible had been hit, saying, "A number of Argentine naval aircraft attempted to attack the carrier battle group this afternoon. None of our ships was hit."

One Argentine Skyhawk fighter-bomber was believed to have been shot down by a British destroyer, according to the ministry.

The Argentine communique did not name the Invincible, but said the attack occurred about 90 miles east of the islands. The Argentine military is known to believe that the Invincible is the only British carrier in that area.

The communique said the carrier was forced out of action because of heavy damage inflicted by Super Etendard jets, equipped with French-made Exocet missiles--such as the one credited with sinking a British destroyer and disabling a cargo vessel--and A4 Skyhawks with "high-powered bombs." Skyhawk pilots reported dense columns of smoke and fire from the carrier, the military command said.

The communique called the attack an "unprecedented" operation.

As the Argentines shored up their long-prepared trench line at the Falklands capital of Stanley, the military joint chiefs of staff conceded in a communique tonight the "establishment of the enemy in the Darwin-Goose Green area" and said British ships had bombarded Stanley early this morning. The military command reported that British Harrier planes later attacked the island's capital and that two of them were shot down and a third damaged.

While conceding that Argentine forces had lost control of the area around Darwin and the strategic airstrip at Goose Green, military officials in Buenos Aires maintained today that British forces had sustained heavy losses of men and materiel in the last days of fighting and were being harassed as they advanced across the island.

Argentine commanders deliberately had left only small garrisons at Darwin and other positions on the islands, officials said, so as to not weaken the main defenses at Stanley, which Argentina calls Puerto Argentino. Military officials maintained that harassment operations by Argentine troops and air strikes on supplies and ships in the past week had weakened British forces enough to prevent a successful storming of the main Argentine garrison.

The military command issued an earlier communique saying that Argentina had suffered a total of 82 dead, 106 wounded and 342 missing since its invasion of the Falklands April 2. This figure apparently included 321 missing and presumed dead from the sinking of the cruiser General Belgrano as well as losses in air raids and on the ground.

In an apparent effort to soften the impact of the report, military officials claimed that their unofficial tallies showed 572 British casualties during the past week alone.

Officials said today that the military command hoped to reinforce the 7,000 troops reported to be dug in around Stanley while attempting to slow the British advance. Reports here said it is still possible for Argentine planes to slip into the Stanley airport with supplies from the mainland, although such flights have become increasingly sporadic and risky.

Argentina's military leadership now appears to have little hope of driving the British off the islands with air raids or a ground counterattack, analysts here say. Rather, the military hopes to stop the British in front of Stanley and inflict enough damage to force a cease-fire and a negotiated settlement. Government officials said they did not believe serious negotiations with Britain were now possible unless the British move on Stanley was convincingly checked.

Foreign Minister Nicanor Costa Mendez, returning to Buenos Aires from Washington late last night, told reporters that "there is still a lot to negotiate, a lot to discuss." But he added, "There is also a lot to resolve on the battleground, where we should repel the British forces."

Costa Mendez joined in a meeting of the ruling military junta tonight to discuss Argentina's diplomatic position, including a possible trip by Costa Mendez to a meeting of nonaligned nations taking place in Cuba.

Despite the reverses of the past 10 days, military officials maintained that they were still confident that Stanley's defenses would hold. Argentine forces have now had more than eight weeks to prepare their positions there, they noted, and are equipped with heavy artillery and antiaircraft defenses that had been only thinly distributed in other parts of the islands.

The waters around Stanley's protected harbor have been mined to ward off amphibious carriers launched from British warships, according to reports here. On the inland front, Argentine forces are dug into ridges or aligned in deep bunkers that British forces will have difficulty overrunning because of terrain that is alternately steep and rocky or marshy.

Officials here have maintained that Argentina's mainland-based Air Force also retains the strength to launch a powerful attack on British warships approaching the island or even on advancing troops. But analysts here said Argentina would probably have to depend more on its ground forces to protect Stanley in the coming days because of the difficulties of air operations against British troops on the move.

As weather conditions have grown worse, Argentine planes have had trouble launching operations from bases 400 miles from the islands and often have been prevented by cloud cover from pinpointing British targets. Reports here said today that Argentine planes were unable to operate during most of Friday's fighting around Darwin and Goose Green because of poor weather.

Military officials were also quoted as saying that Argentine forces had fought at Darwin until they ran out of ammunition and could not be resupplied from Stanley. Last night, the military command said Argentine forces totaled only 700 and were outnumbered by more than 3 to 1 by the British. Those numbers flatly contradict Britain's account, which claims 700 paratroopers overcame 1,500 Argentine defenders.

Argentine military officials appear determined to carry on the conflict with Britain even if the Falklands are lost.

The thinking in high military circles seems to be represented by a statement published today from former president and Army commander Alejandro Lanusse. He said there was a difference between "the battle of the Malvinas," or Falklands, and the "war against Great Britain."

"Even imagining the worst," Lanusse said, "that they throw the last of our men out of the Malvinas, we will have lost the battle, but not the war . . . . The battle will have ended and then we will continue the war, the real war."