British ground forces, advancing across the marshlands of East Falkland Island toward the capital of Stanley, reported yesterday that they took 900 Argentine prisoners in Friday's capture of the strategic Goose Green airstrip.

The British Defense Ministry in London also announced that the commander of the paratrooper battalion that took Goose Green and neighboring Darwin, Lt. Col. Herbert Jones, was killed in the assault, the first major ground combat of the two-month-old conflict.

The ministry gave no other account of casualties and did not comment on reports from British military sources that Royal Marines had launched assaults on Argentine outposts at Douglas and nearby Teal Inlet, 25 miles east of the British beachhead at San Carlos and a similar distance west of Stanley.

Argentine President Leopoldo Galtieri, in a speech in Buenos Aires, said his forces were fighting an "unequal battle" against Britain and "those who support her." But Galtieri pledged Argentina "will continue fighting the enemy for each piece of Argentine soil" and warned that his country would obtain military aid "from other parts of the world" if necessary.

The Argentine leader, suggesting that the United States could be blamed if Argentina were defeated, denounced "certain governments that subordinate their proclaimed principles to dark interests and dubious compromises." [Details on Page A19.]

Argentina's military high command, indirectly acknowledging the loss of Goose Green and Darwin, said late last night that its communications had been severed with troops defending the two settlements.

"Argentine troops, though amply surpassed in terms of men, firepower and mobility, resisted the attack," a communique said. "At midday, radio contact with them was lost, and it has not been possible to renew it so far."

Noticias Argentinas, an independent news service, quoted high military sources as saying the Argentines, outnumbered 3-1, "ran out of ammunition during tough combat before surrendering their positions."

The Argentine command "preferred not to waste its reserves in the Darwin battle in order to maintain its principle forces at Puerto Argentino Stanley ," the military sources were quoted as claiming. They added that the British Marines were "surprised by the tenacity of the Argentine conscripts in combat" and that the Argentine Air Force, which so far has been Argentina's main strength against the British, could provide little support because of bad weather.

Military sources in London were reported as saying the Royal Marines, forming the northern arm of a two-pronged advance, took about 200 Argentine prisoners at Douglas and Teal Inlet after fierce fighting.

Meanwhile, the southern arm of the offensive, the paratroopers who had taken Goose Green and Darwin, reportedly turned east and began moving along the 50-mile unpaved road that leads to Stanley, where the major Argentine garrison--now estimated by British sources at 4,500 to 5,000 men--is dug in.

The paratroopers, backed by Scorpion light tanks and Land Rovers, must negotiate the bog-ridden road and face the likelihood that Argentine defenders, positioned along a 2,300-foot ridge, will resist their advance, military sources said.

The reported capture of the 900 soldiers, about 10 percent of the total estimated Argentine garrison on the Falklands, was considered by analysts in London to be a major blow. It suggested that the estimated 5,000 British troops ashore on the Falklands, combined with the reported 3,500 soldiers shipped to the war theater aboard the liner Queen Elizabeth 2, may be within reach of combat superiority in numbers over the Argentine forces.

There were unconfirmed reports in London yesterday that the additional force, which includes Welsh and Scots Guards and Gurkhas, already had been transferred from the QE2 to assault ships. Analysts speculated that the troops may be poised for a landing somewhere near Stanley to establish a second British beachhead in an effort to surround the Argentine defenders.

British Defense Ministry spokesman Ian McDonald said a detailed statement on British casualties would be released after families of the dead and wounded had been contacted.

Military sources said Lt. Col. Jones was cut down as he and a small squad of men stormed an Argentine machine-gun nest defending the airstrip, American news agencies reported. Jones, a 22-year veteran of the British Army, was the highest ranking serviceman known to have been killed in the Falklands.

The Defense Ministry also reported that one Argentine Pucara jet was shot down Friday over Goose Green.

Earlier, Argentine officials appeared to be preparing the public for a possible military defeat on the Falklands, which Argentina seized from Britain April 2. A military communique acknowledged that Britain has landed between 4,000 and 4,500 troops--instead of 2,000 as previously claimed--and that the British had "consolidated" the beachhead at San Carlos.

There was no diplomatic activity at the United Nations yesterday, where Secretary General Javier Perez de Cuellar has launched a new round of talks seeking a peaceful resolution of the conflict.

Despite the reported triumphs of the last two days, British military sources conceded that their forces face a tough fight to recapture Stanley from the entrenched Argentine defenders. One resident who journeyed to the British lines from the capital was quoted by reporters with the naval task force as saying the town "was swarming with thousands" of Argentine troops.

"They are very edgy," said Patrick Minto, 20. "The young soldiers just think they see something move and they let fly. They have shot up a few houses because they thought they saw something move."