Argentina is scouring the world "in a massive scramble" for military supplies ranging from French Exocet missiles to field uniforms. At the same time, Britain has requested, and received, stepped-up shipment of American arms, including Sidewinder air-to-air missiles, for the escalating war in the Falkland Islands, administration officials said yesterday.

Both sides, these officials said, are trying to beef up their forces as they gird for the biggest battles of the war, including the possibility of bloody land combat as they fight for control of the key city of Stanley on East Falkland. The Argentine effort, knowledgeable sources said, approaches desperation. It is focused on persuading businessmen from Peru and Venezuela to search everywhere for advanced weaponry, like Exocet missiles, guns, ammunition and even field uniforms.

"All kinds of middlemen are at work," said one official of the Argentine scramble, "and the prices are high."

Since France had promised not to sell Argentina Exocet missiles or other weapons directly, the officials said, the middlemen are concentrating on countries with such hardware on hand. Besides Argentina's South American neighbors, these middlemen are approaching countries as distant as South Africa in this hurried search for arms that could be rushed into the battle of the Falklands, sources said.

However, administration sources flatly denied that there is any evidence in U.S. hands confirming reports that Edwin P. Wilson, an international weapons dealer based in Libya, is working for Argentina as part of the scramble.

U.S. officials studying fragmentary reports on Argentina's great weapons hunt said no massive shipments of arms seem to have been completed in recent days. "They're scrambling everywhere trying to buy," said one official.

As for Britain, defense officials confirmed yesterday that Britain has requested and gotten hurry-up shipments of the Air Force's AIM9L Sidewinder missile for aerial combat.

The AIM9L would give the British Sea Harrier jump jets a big edge over Argentina's fighter planes. The missile, unlike ones the Argentines have, does not need to be fired into the side or rear of the target plane to make sure it homes in on the engine heat but can be launched from any angle, including head-to-head.

Although Britain has been buying AIM9Ls right along, the Pentagon acknowledged that shipments have been stepped up since the war over the Falklands began. The same is true, they said, of Hawk antiaircraft missiles and steel landing mats for building temporary airstrips on the Falklands for British Harriers and helicopters.

This U.S. tilt toward Britain in selling weaponry is being hotly protested by Argentina and its South American supporters.

The weaponry being speeded from U.S. inventories to the British represents a triple threat to the land-based aircraft Argentina has relied on so heavily in the war.

The AIM9L Sidewinders would help British Harrier pilots knock down Argentine fighter-bombers before they got within range of the fleet; the mobile Hawk missiles on the ground would put a protective umbrella over British troops trying to widen their beachhead, and the steel mats laid out on the islands would enable the Harrier jump jets to fly farther than they could from ships.

Although Harriers can take off straight up, they must go light in this mode because there is no roll down a runway to generate air flow to lift the wings. The British have put "ski jumps" for their Harriers on ships to provide some of the desired roll. But steel mats spread out on land would do much more toward extending the Harriers' range and enabling them to carry a bigger load of bombs or missiles.

Pentagon officials have been reluctant to disclose what weaponry the United States is sending Britain. Pentagon spokesman Benjamin Welles said one reason for this is that the administration was worried about what might happen to Americans in Argentina as a result of "inflamed public opinion there about U.S. backing of Britain against Argentina."

Although Welles did not say so, another reason could well be the Pentagon's desire to salvage as much as possible from the U.S.-Argentine military relationships built up before the Falklands war. Defense leaders saw those relationships as a big help in helping to stabilize South America.

Meanwhile, Washington Post correspondent Jackson Diehl reported from Buenos Aires that military sources confirmed that Argentina was actively seeking new supplies for its air force in Latin America and in other Western arms markets.

In the past, Peru, Bolivia and Venezuela have hinted they would provide military aid to Argentina under some conditions. Peruvian President Fernando Belaunde recently said, however, that Peru did not want to destroy its chances of mediating by backing Argentina militarily.

Sources in Buenos Aires said that the ruling junta had decided at the onset of the conflict with Britain not to accept arms from the Soviet Union or other members of the East Bloc. So far, this policy has not been altered, the sources said, despite hints from military officials in recent days that Argentina could turn to the Soviets.

Argentine newspaper reports said that the air force had already been promised new supplies of arms from third countries, including fresh planes. A report in the afternoon paper La Razon, quoting a reliable military source, said that the new Mirage III, and Israeli-built Mirages called Daggers, would shortly be arriving at southern air bases from "friendly countries."

This report was not confirmed by military officials, who have often said that unofficial reports appearing in Argentina are part of the "psychological warfare" between Argentina and Britain.