The big challenge ahead for the British navy is not setting up a blockade of the Falkland Islands but sustaining it, U.S. military specialists said yesterday.

The warships and submarines the Royal Navy could deploy around the islands about 300 miles off Argentina have the firepower needed to sink or disable ships caught trying to run the blockade, they agreed.

However, the British aircraft carriers and destroyers headed for the Falklands run on oil and will need to be refueled constantly after they reach their destination. This poses unusually tough problems, primarily because there are no British ports in the vicinity.

Pentagon officials said yesterday that they expect the Royal Navy to turn Ascension Island in the South Atlantic into a supply house for the current emergency, but that rocky dot on the ocean is still about 4,000 miles from the Falkland Islands.

It would simplify things greatly for the British flotilla if some country near Argentina, such as Brazil, would allow its ships to refuel at its ports. But there is no sign of this happening, leaving seagoing gas stations traveling with the task force as the alternative.

But even such tankers full of fuel for the British ships, AV8A Harrier jump jets and helicopters will run out of fuel if the blockade stretches from days to weeks, raising the question of where they will go to fill up. Buying fuel from a commercial tanker and taking it on at sea is one possibility; asking U.S. Navy oilers for help is another.

Asked yesterday whether the British had requested help from the United States in keeping its Falklands-bound task force supplied, Pentagon spokesman Henry E. Catto Jr. replied that he was "not aware of any requests for logistics assistance."

He said the 1962 agreement under which the United States built a 10,000-foot-long runway on the British-owned island of Ascension "carries with it the obligation of the United States to provide fuel needed for safe continuation of flight of the aircraft within the limits of our existing capability at the airfield."

Pentagon officials said there also is a dock at Ascension where a limited number of British ships could tie up and refuel if the British manage to stockpile fuel there--again a difficult task since the island is 4,000 miles from Britain. Besides fuel, Britain must supply its sailors and Marines with food and ammunition.

The strain of doing all this at the end of an 8,000-mile supply line was dramatized by the British announcement that it has requisitioned two civilian ships to support its Falkland-bound task force, the 45,000-ton cruise liner Canberra and the 8,500-ton container ship Elk.

The umbilical cord that keeps conventionally-powered ships tied to land supplies of fuel, food and ammunition for extended conflict does not extend to the nuclear-powered British attack submarines which are expected to be the first to reach the Falklands.

British Defense Minister John Nott said yesterday that starting Monday morning any Argentine navy ships that come within 200 miles of of the Falklands will be sunk. At least two nuclear submarines are expected to be in position to try to carry out that threat.

There is a problem with using submarines to enforce a blockade, however. Although nuclear subs could outrun Argentine warships if detected, and thus would not stand much risk of being sunk by them, they would expose themselves to Argentine aircraft if they surfaced to warn away a ship trying to run the blockade.

The subs could sink such a ship without surfacing, but this might provoke worldwide protests against the British since the usual blockade practice is to warn before resorting to sinking the violator.

High officials said there are no signs that Soviet submarines are moving toward the Falklands. The Pentagon also denied yesterday a television report that the United States had sent the SR71 Blackbird spy plane over the Falklands.