Senior administration officials last night confirmed that the United States is providing assistance to the British fleet sailing to the South Atlantic and to Royal Navy submarines already near the Falkland Islands.

The officials said the British have requested and received from the United States the use of a channel on a U.S. communications satellite to help London communicate with their submarines patrolling off the Falklands and with the surface armada still more than 3,500 miles from the region.

Availability of such improved submarine communications could be a tool for avoiding inadvertent confrontations as well as for providing help in a clash.

Officials also confirmed that U.S. intelligence surveys of Argentine military activity have been passed to London. These reportedly include photographic evidence gathered before Argentina's invasion of the islands, and suggested that an invasion might be coming.

Officials also confirmed that fuel tanks on Ascension Island in the Atlantic have been topped off with about 2 million gallons of jet fuel for potential use by British warplanes.

Under a joint-use agreement signed in 1962 the United States is obligated to make available Ascension's airfield and fueling capabilities to the British upon request. Ascension is Britain's closest land supply point to the Falklands.

The United States also has supplied weather forecasting information for the Falklands region, although officials said the information generally is available internationally. Heavy clouds and gale-force winds are not unusual near the Falklands at this time of year, so accurate forecasting could be extremely important to military operations.

Officials' confirmation of the information, first reported by ABC-TV news last night, raised questions about how it might affect efforts by Secretary of State Alexander M. Haig Jr. to serve as a go-between trying to resolve the Falklands crisis.

The Reagan administration has supported Britain by voting for a U.N. resolution condemning the use of force by Argentina and calling for Argentine withdrawal from the Falklands.

But the administration also has described both countries as friends, has taken no position on differing claims of sovereignty over the islands and has sought to remain neutral in order to serve as an effective go-between.

In a related development, other officials said last night that there is no evidence to support an NBC-TV report that the Soviet Union is providing Argentina with intelligence information on British fleet movements.

A well-informed Pentagon official said "some people were jumping to conclusions" about the Soviet role.

Another senior government specialist said that, while the Soviets routinely gather intelligence on all major western naval movements, Moscow has not launched a "major" intelligence-gathering effort on the Falklands situation.

The NBC reports said Moscow has taken the unusual step of providing Argentina with highly sensitive information gathered by satellites, long-range reconnaissance planes and ships.

Informed officials, however, said Moscow does not have a satellite in position to provide surveillance of the southernmost reaches of the Atlantic.

Soviet TU95 Bear reconnaissance planes based in Cuba and Angola have flown over the British fleet in what some officials describe as routine tracking of the fleet of a major member of the North Atlantic Treaty Organization.

Other officials said they believe Moscow might turn over that information to Buenos Aires. However, all officials interviewed last night said there is no indication that such information has been passed to Argentina.

Soviet naval activity in the mid-Atlantic is said to involve two frigates operating near Angola and three supply ships making normal ocean transits.

Soviet fishing trawlers equipped for intelligence operations are shadowing the British fleet along the route to the Falklands. About 30 Soviet and East European fishing ships are in the Falklands area, a relatively normal complement in the fishing-rich area.

Senior analysts said it is too early in the conflict for such intelligence to be of much help to Argentina. As of last night, these sources said, the British fleet had not reached Ascension Island, 3,500 miles from the Falklands.

When the fleet nears the Falklands, Soviet intelligence could be important to Argentina if Moscow decides to make it available.

For example, officials say it would be helpful for the Argentines to know the position within the fleet of ships equipped with surface-to-air missiles meant to defend British aircraft carriers against attack by Argentine aircraft.