President Leopoldo Galtieri warned today that Argentina is prepared to fight for years and suffer tens of thousands of casualties to defeat Britain in the South Atlantic.

In an interview with a Mexican television station, the Argentine leader predicted that "in the coming days it is probable" that there will be some kind of a narrowing of the gap between the Argentine and British negotiating positions over the Falkland Islands. He described this possibility as "not definitive" but rather an attempt "to look for a solution" in the six-week-old crisis.

But Galtieri, a general who is also the Army commander-in-chief, coupled this vague optimism with a pledge that in order to defend its claim on the Falklands, Argentina was willing to fight "five or six months, or five or six years," and suffer "4,000, or 40,000 more" casualties.

"At this moment, I have on my shoulders the spilled blood of more than 400 Argentines," he declared. In official statements, the Argentine Army and Air Force have acknowledged 53 dead and 62 wounded in fighting on the islands, while the Navy has listed 20 dead and 301 missing from the sinking of the cruiser General Belgrano May 2. Britain has put its dead in the hostilities at 24.

The interview came as government leaders huddled to prepare what appeared to be a detailed response to U.N. proposals for a diplomatic settlement with Britain. Officials here appeared to be preparing public opinion for a possible new breakdown of the negotiations.

Argentina's military command, meanwhile, reported in a communique that "an enemy surface unit" had bombarded Port Calderon, which lies on Elephant Bay in Pebble Island--called Bordon Island by the Argentines--just northeast of the island of West Falkland. The attack took place at 3:50 a.m. EDT, the communique said, and damaged three Argentine planes on the ground. The report did not mention British claims that commandos had landed and later had withdrawn from the island and gave no accounting of casualties.

Until now, British assaults on the Falklands have focused on the main island of East Falkland, where the capital, Stanley, and Darwin are located, along with the main Argentine troop position and airstrips. British forces irregularly bombarded positions on East Falkland earlier this week, and carried out air attacks on Stanley yesterday.

British forces also have said they fired earlier this week on an Argentine vessel located in the narrow sea channel between East and West Falkland.

An Argentine communique said efforts to recover a cargo boat reported missing in the area yesterday had been "unfruitful."

The communique said the 3,900-ton cargo vessel Isla de los Estados, a merchant ship with a civilian crew, was resupplying food, medicine and fuel to the Falklands, which Argentina calls the Malvinas. A naval official unofficially estimated its crew at between 40 and 50.

Argentina has launched only one attack of its own during a week of British operations, an air raid Wednesday against British warships by squadrons of American-built A4 Skyhawks.

There were widespread reports here today that a British frigate had sunk as a result of the attack. London has said that a frigate suffered only minor damage.

The report in principal morning newspapers, citing military sources, said one of two British frigates guarding the aircraft carrier Invincible had been struck by a 1,000-pound bomb and had since sunk.

Argentina has conceded losing two Skyhawks in the attack, and an Air Force statement said two pilots had been killed.

After three weeks of sporadic fighting around the Falklands and on South Georgia Island, Argentina has now formally conceded losing the cruiser General Belgrano, the submarine Santa Fe, the tug Narwal, four aircraft and a helicopter. Four aircraft and a second tug, the Sobral, have been reported damaged, and the cargo boat Isla de los Estados remains officially missing.

Argentina claims to have downed nine British Sea Harrier jets and a helicopter, and to have damaged various ships. Unofficial reports have claimed the sinking or disabling of a half dozen British warships, including the flagship aircraft carrier Hermes.

Britain has announced the sinking of the destroyer Sheffield by an Argentine missile attack May 4, although Buenos Aires has never claimed it. Argentine sources have said the military was never sure that it had hit the Sheffield rather than another British ship. In addition, analysts say that by failing to confirm the Sheffield report, Argentina may be trying to cast doubt on all British reports.

The television interview was the third Galtieri has given to foreign journalists this week after a period of relative seclusion with the two other members of the ruling junta. His outspokenness this week has been seen here as part of a widespread public relations campaign to strengthen Argentina's international support by showing flexibility and suggesting that it is the British who are intransigent in diplomatic negotiations.

Foreign Minister Nicanor Costa Mendez told reporters last night that the recall of British U.N. Ambassador Anthony Parsons to London from talks in New York with U.N. Secretary General Javier Perez de Cuellar "shows that those that are delaying the negotiations are not the Argentines but the British."

Costa Mendez and military representatives were expected to meet tonight to discuss the U.N. mediation effort, government officials said, and a meeting of the military junta has been planned for Sunday to finalize Argentina's position.