Darwin, believed to be the most important Argentine base in the Falkland Islands after the capital of Stanley, has been a principal objective of British air attacks since the task force began operations against Argentine bases there May 1.

A large part of Argentina's ground forces are believed to be based there, along with light aircraft based at a nearby grass landing strip. The loss of Darwin would leave Argentina with only Port Stanley as a defensive stronghold well-supplied with artillery and fortifications, military analysts here said.

Although military officials here continued to claim late today that an Argentine counterattack had halted the British operation, the news of the ground attack on Darwin today indicated that Argentina in any case had failed in its attempt to keep British forces pinned down to the 90-square-mile area in the northern part of the island where the first landing was made a week ago.

During the week, Argentina has reported launching almost daily air attacks on the British camp, hoping to destroy fuel supplies, drive off ships attempting to resupply the area and prevent British construction of a strip for planes.

The military command reported yesterday that British Harrier planes had attacked Darwin, Port Stanley and Port Howard, which lies across the Falkland Sound from Darwin, on West Falkland Island. The military also reported Argentine air and ground attacks against the British base, including a raid by Canberra bombers, but they have offered no information on the results of those operations.

Early this morning, the military command issued a second warning on the British hospital ship Uganda, which it said was being used for military purposes in violation of the Geneva Convention. It asked that the vessel be removed from waters near the British land base, saying Argentina would not be responsible "for the events which could occur."

There were no reports of major Argentine air strikes on the attacking British forces by late today, but officials said an Air Force attack was likely if British ships moved in to support ground troops attacking Darwin or Port Stanley.

Navy Commander-in-Chief Jorge Anaya flew to Ushuaia, at the southern tip of Argentine territory, today after an inspection tour yesterday at the key southern base of Comodoro Rivadavia. His trip prompted new reports that the Argentine Navy, including the aircraft carrier 25th of May, would soon join the defense of what was perceived here as a major British operation.

One military official said today that Argentine vessels are believed to have been stalked by several of Britain's nuclear submarines, which are expected to attack the aging aircraft carrier as soon as it leaves coastal waters. The official said, however, that the Navy had decided to use its 11 major warships "at whatever cost."

President Leopoldo Galtieri remained in his office today after conducting an unusual five-hour meeting last night with the 10 top generals in the Army, of which he is commander in chief. With Anaya in the south, there was no indication that the ruling junta was yet considering the new round of diplomatic negotiations opened in New York by U.N. Secretary General Javier Perez de Cuellar.

While the military command maintained silence on the ground battles today, the state news agency Telam reported that southern airbases had received new supplies of American-made A4 Skyhawk fighter bombers and Argentine-made Pucara planes. The agency did not say whether the new A4s had been obtained abroad, or whether they were part of a reserve force.

The agency reiterated reports here that the Argentine Air Force was not suffering from a shortage of pilots because of its losses during the last week. It maintained that most of the pilots shot down have been rescued. Argentina has officially acknowledged the loss of only 17 planes since the conflict with Britain began, but unofficial reports here have quoted military officials as conceding the number may be significantly higher.

Telam also reported from Port Stanley, which Argentina has renamed Puerto Argentino, that civilian residents of the islands could be suffering from food shortages and that some of the 600,000 sheep in the territory had been rounded up to provide meat.

The agency quoted Argentina's Falklands commander, Gen. Mario Benjamin Menendez, as saying the supply situation of the islanders was "evidently difficult" as a result of the British naval and air blockade.

"If the islanders don't receive the necessary elements for their survival, it is the exclusive fault of the English, who have never bothered with the fate of these residents," Menendez was quoted as saying.