Britain's South Atlantic task force launched a major landing operation southwest of the Falkland Islands capital of Stanley today in what Argentine military officials said appeared to be the beginning of a British offensive against Argentina's last stronghold on the disputed islands.

British troops landing at Port Fitzroy, about 12 miles southwest of Stanley, came under heavy attack from Argentine planes, and Argentine military officials said tonight that the warplanes had destroyed a British frigate and hit three landing ships.

The officials also reported that British troops surrounding Argentina's garrison of about 7,000 troops at Stanley had carried out three probing attacks on Argentine lines during the day and that the two sides had exchanged artillery fire.

In a short, late-night announcement in London, the British Defense Ministry disputed the Argentine claims that the frigate had been destroyed, but acknowledged that three ships were damaged, Washington Post correspondent Leonard Downie Jr. reported. The Defense Ministry said that the attacking Argentine aircraft suffered heavy losses.

The ministry said without elaboration that the three ships--the logistic landing ships Tristram and Galahad, and their protecting frigate Plymouth--had "suffered some damage." It added that "initial reports are that five casualties from the Plymouth have been removed to another ship and are receiving treatment." It provided no information on any other British ships involved.

The British said that "at least two Argentine aircraft were shot down," during the attack. "A further one was possibly shot down, and at least four others were damaged--two of these were seen trailing smoke," the ministry spokesman said.

No questions were taken at the British briefing, Downie reported from London, and no further announcements were expected during the night, apparently to avoid revealing the overall strategy of British forces.

According to the British Defense Ministry statement, the landings reinforced "elements of the Fifth Brigade" which "are now firmly established at Fitzroy Settlement and Bluff Cove." The Fifth Brigade's Scots and Welsh Guards and Gurkhas landed last week after being transferred from the Queen Elizabeth II.

"Over the last few days," the ministry announcement continued, "initial deployments to that area have been reinforced, and the troops and their support are now ready to move forward," presumably toward Stanley. Earlier announcements in London today had put the total British troop strength on the Falklands at 9,000.

Fitzroy and Bluff Cove are just south of the ridge and mountains that had constituted the British front line west of Stanley. Their occupation would provide British forces with an additional southern salient aimed at the Falklands capital.

Although reports from both sides reflected the first major fighting on the Falklands since Britain captured Argentine bases at Darwin and Goose Green May 29, a news blackout today from the islands themselves prevented independent verification of the conflicting claims.

In an official communique tonight, the Argentine joint military chiefs said that the British frigate, which an earlier announcement said had been struck by rockets and bombs, had exploded and rapidly sunk in a bay called Bahia Agradable by the Argentines and Port Pleasant by the British. The communique added that "it can be supposed that there were numerous casualties" among the crew.

It said that two of the landing ships caught fire, had explosions on board and were abandoned by their crews. The third ship, according to the Argentine communique, caught fire and "was left out of action."

"The personnel and materiel that was disembarked also was attacked, causing a high number of English casualties and the destruction by explosions and fire of most of the materiel," the communique said.

An Argentine military spokesman told reporters in a briefing tonight that the British move at Fitzroy began this morning when a frigate and three landing ships carrying troops closed in on the coast. Argentine planes based on the mainland struck the British ships and the landing troops at 12:50 p.m. EDT, the spokesman, Capt. Enrique de Leon said.

Firing rockets and dropping bombs, Argentine planes hit all three British troop ships in the air raid, but the frigate withdrew safely from the zone, according to the Argentine account.

A second wave of Argentine planes attacking at 3:00 p.m. EDT hit the frigate at Bahia Agradable. The planes then attacked British troops landing in the area, inflicting heavy casualties and destroying materiel that had been disembarked, de Leon said.

During the second attack, Argentine pilots saw that the British landing ships attacked earlier had caught fire, de Leon said.

Other military officials said that the British landing force had not been covered by Harrier or Sea Harrier warplanes, thus facilitiating the Argentine air attacks.

De Leon did not say how many Argentine planes were involved in the air raids today, or whether any of them had been shot down.

Reports here tonight also said that Argentine commando units operating behind British lines attacked and destroyed British supply dumps on East Falkland Island. Various reports, quoting military sources, also said Argentine troops had neutralized a British commando unit operating inside Argentine defense lines, capturing or killing its members.

The private Argentine news service Noticias Argentinas quoted military sources as saying that the British had landed as many as 400 men at Fitzroy, about the strength of a brigade.

Meanwhile, Foreign Ministry officials reported that both Britain and Argentina had rejected the latest cease-fire proposal by United Nations Secretary General Javier Perez de Cuellar. Government sources said that Argentine government leaders now had no hope of preventing the long anticipated land battle for Stanley.

As hope of peace seemed to end in Buenos Aires, military leaders strove to assure the country that the conflict with Britain could be won.

President Leopoldo Galtieri, in an interview with a Spanish magazine released here today, said, "Argentina is ready to continue the war for the months and the years necessary." Galtieri added that Argentina had received a number of offers of military assistance from other Latin American countries, and that Argentina "would accept them to the extent that it needs them."

Sources in Buenos Aires maintained that Argentine troops had received fresh supplies in recent days and had taken over houses in the town of Stanley for shelter from winter weather.

One high military source, in an interview, said Argentine troops were expected to hold out for a long time against a British offensive. He said Argentine troops were well supplied, despite the British blockade against air and naval traffic between the Argentine coast and the islands, and had enough food and ammunition to fight for weeks.

The Argentine Air Force, the official said, had reported to the military command recently that it had lost about 30 percent of its fighting power during the last month of fighting. This figure is far higher than that officially acknowledged by the Argentine military command, which has reported losing fewer than 20 of its more than 120 high performance aircraft.

Argentine diplomatic sources said here that Argentina is not now considering the possibility of a diplomatic solution until after extensive fighting has taken place around Stanley. Foreign Ministry officials are said to believe that the next chance for a settlement of the conflict could come if British forces are stalled in an offensive, thus creating the possibility of a cease-fire in which U.N. troops would separate the two sides, allowing both armies to remain on the islands while negotiations continued.