Britain said its forces expanded their hold on the Falkland Islands yesterday while both London and Buenos Aires reported only scattered fighting in the disputed territory. The British Defense Ministry acknowledged the loss of a frigate during Friday's fierce battles.

Additional troops landing yesterday brought the total of British forces on the Falklands to about 5,000, high-ranking military sources said. The number of Argentine troops on the islands has been estimated at 9,000.

Defense Secretary John Nott said at a news conference that Argentine jets had sunk the 4 1/2-year-old frigate Ardent on Friday. British military sources said four other warships also were damaged, and 20 seamen from the five ships plus five other combatants were dead or missing. This brought the reported British death toll to 70 since Argentina invaded the British colony April 2.

Argentine President Leopoldo Galtieri told reporters in Buenos Aires that the British have "managed to gain a foothold in the San Carlos zone" of East Falkland Island, but said Britain had suffered "enormous" losses of "lives and materiel." The joint chiefs of staff said fighting was continuing around the British beachhead, but maintained that Argentine forces had the situation "under control." No estimates were given of Argentine casualties.

Galtieri said Argentina is still interested in negotiations "without renouncing the sacred interests of the Argentine nation, which come from the depths of our history."

Argentine Foreign Minister Nicanor Costa Mendez arrived in New York to present his country's case at the U.N. Security Council debate in which the Soviet Union, Spain and several Latin American nations called for a cease-fire and the resumption of U.N. Secretary General Javier Perez de Cuellar's mediation efforts.

In Rome, Pope John Paul II prayed for peace with cardinals from Britain and Argentina in St. Peter's Basilica, and sent telegrams to the leaders of the two nations urging them to do everything possible "to avert further sacrifices and bloodshed."

The two British cardinals who attended the mass with the pope said when they returned home that the pontiff planned to go ahead with his visit to Britain Friday unless the local bishops advise him not to. Details on Page A19

The U.S. Embassy in Buenos Aires stepped up evacuation of diplomats' families and "nonessential personnel."

Conflicting accounts came from the two capitals of virtually every aspect of Friday's confrontation and the current relative position of their forces. Nott said British troops had established a 10-square-mile beachhead on San Carlos and were advancing beyond it. He said they would "advance to place the occupying Argentine troops under increased harassment, whilst the Royal Navy maintains and tightens its blockade around the islands."

The official Argentine news agency Telam said Argentine artillery was pounding 600 Royal Marines trapped on seven square miles of beach at San Carlos. It said they were the only British forces known to be on the islands.

The Argentine high command said three British Harrier jump jets and two helicopters had been shot down, and Britain had downed six Argentine planes and damaged three helicopters.

Nott said Britain shot down 16 Argentine planes--nine Mirages, five Skyhawks and two Pucaras--and four helicopters. "These losses," he said, "must represent a very signficant blow to the Argentine Air Force."

Britain said its forces had lost two helicopters and one Harrier. In addition to the sinking of the Ardent, Nott said four other British warships were damaged in Friday's Argentine attack on the British units.

There were reports of only sporadic fighting on the islands yesterday. The private Argentine news agency, DYN, quoted Air Force sources as saying Argentine plans did not attack the British force yesterday "for lack of worthy targets."

The only significant confrontation reported by the British Defense Ministry was an attack on an Argentine patrol boat by two Harriers on routine patrol south of Stanley. The patrol boat was severely damaged, the British sources said.

Asked about Argentine statements that the British invaders were being "cleaned up," Marine Lt. Col. Tim Donkin said at a London news conference that the British attack "has not been repelled, nor will it be. We are there to stay."

Donkin said the British task force had more than 100 ships and 25,000 men in the war zone.

The Ardent was the second British warship sunk during the Falklands fighting. The destroyer Sheffield was demolished and 20 of its crew lost after it was hit by a missile from an Argentine plane on May 4.

Two days earlier, a British submarine torpedoed Argentina's only cruiser, the General Belgrano, and Argentina later reported that 321 seamen were killed. Argentina has announced a total death toll of about 400.