U.S. intelligence officials instructed spy satellites to provide better coverage of Cuba for photographic evidence of increased Soviet military presence, sources said yesterday.

A satellite can be adjusted to take a varying number of photographs of several countries as it whirls around the earth. But it would overload the system to photograph everything the satellite "sees" below.

In the case of Cuba, sources said, intelligence executives settled for routine satellite coverage of the island until a radio intercept indicated unusual military activity.

In response, the Central Intelligence Agency, which runs the spy satellite program, intensified the photographing of Cuba at the expense of some other targets for the cameras.

Unlike spy satellites of an earlier era, the new ones do not have to drop packets of film to be snatched in midair by specially rigged aircraft. Instead, they radio pictures-- something like newspaper radiophotos-- to a special ground station in the United States.

It was these photographs that stunned U.S. intelligence officials last month. Until then, the assumption was that Cuba had only about 2,000 Soviet military advisers on the island but no sizable contingent of combat troops.

But photographs of pup tents, armored units and other evidence of Soviet troops on maneuvers persuaded most intelligence executives-- but not all-- that the Soviets had as many as 3,000 combat troops in Cuba in addition to the 2,000 military advisers and 6,000 to 8,000 civilian advisers.

Although analysts are restudying photographic and electronic intelligence on Cuba, the State Deparment said Friday that "we have been able to confirm" there are "2,000 to 3,000" Russian troops in a combat contingent that "includes armored, artillery and infantry elements."

Members of Congress are expected to demand explanations from Carter administration leaders this week of how 3,000 Soviet combat troops could go undetected on Cuba for so long. Elements of the Soviet combat outfit started going into Cuba in 1976, the State Department has said.

Another key question is why the Soviet Union sent the 3,000-man group to Cuba. Some intelligence officials believe it is part of Russia's payment to Cuba for sending Cuban troops to fight in Africa.

Not only do Soviet troops help steady Fidel Castro's military establishment by providing replacements on Cuba's home ground, some U.S. officials theorize, but the manuevers detected by the spy satellites provide Cuban troops training in Soviet armored, artillery and infantry units.