Argentina said early today that two British warships had drawn within 54 nautical miles of South Georgia and that Argentine troops were prepared for an attack on the isolated, thinly defended island.

The Foreign Ministry released a note delivered by Argentina to the Organization of American States in Washington saying that as of Wednesday night, the "colonialistic British fleet" was still more than 3,000 miles northeast of the Falkland Islands, seized by Argentina three weeks ago. Two ships apparently moving ahead of the main fleet were approaching Grytvika on South Georgia, a dependency of the Falklands 800 miles east of them, the note said.

South Georgia has been considered a likely first objective of the British naval forces advancing in the South Atlantic. The mountainous, icy island is nearly uninhabited and is out of the range of Argentine air forces based on the mainland. Widespread reports here yesterday said that the island garrison was being reinforced and that heavy artillery had been positioned along landing sites, but only 300 to 400 troops were reported to be on the island.

Argentina invaded South Georgia one day after its invasion of the Falklands. Since then, Argentine officials have repeatedly said that the island will be defended with force.

"We have a desire for peace in our hearts," said Navy Commander in Chief and junta member Jorge Anaya in Buenos Aires, "but hot powder in our cannon."

While Britain has refused to state the location of any of the estimated 40 ships in its flotilla, a senior defense official quoted by United Press International in London said the advance ships were "in the vicinity" of South Georgia, where they encountered 85 mph winds and 40-foot waves.

Britain has made no effort to curb speculation that the fleet will arrive in the Falklands area this weekend.

A dispatch by the Guardian's defense reporter, who is considered to reflect the ministry's thinking, reported: "All the signs are that if last-minute diplomatic maneuvers fail, the British government has set Monday as the deadline for using military force."

In London today, as Prime Minister Margaret Thatcher awaited the return of her foreign secretary from talks in Washington, she made a strong public point of visiting the Admiralty, news services reported. At the same time, the British Broadcasting Corp. issued its most insistent advisory asking the estimated 17,000 British residents in Argentina to leave, "now that the British task force is approaching."

Despite the reports today, U.S. officials in Washington have indicated that supply lines to the faster ships in the British armada are still too uncertain for any sustained action.

The Argentine note delivered to the OAS yesterday said the British had penetrated the zone covered by the Inter-American Treaty of Reciprocal Assistance, or Rio Treaty, and denounced the "proposition of aggression" that the fleet's position indicated.

A special meeting has been scheduled Monday of 21 nations that are voting signatories of the treaty, and Buenos Aires is expected to introduce a resolution on the crisis. Officials said earlier this week that Argentina would not seek joint defensive measures or sanctions against Britain at the session, but Foreign Ministry officials indicated that position would change in the event of a British move against South Georgia.