A U.S. SR71 Blackbird spy plane flew over Cuba Friday to monitor the activities of Soviet troops stationed there.

Sources said the flight, the first over Cuba in nearly a year, covered the area where the Soviets have 2,000 to 3,000 troops, which President Carter last week called a combat brigade but the Soviets referred to as merely a training center unit.

The SR71 flies at 100,000 feet, carrying a wide array of highly sophisticated sensors and cameras capable of photographing broad areas.

Recent revelations that the Soviets have about 3,000 men with 40 tanks and other equipment stationed southwest of Havana caused considerable concern in U.S. administration and congressional circles and threatened to block Senate ratification of the strategic arms limitation treaty with the Soviet Union pending on Capitol Hill.

Carter last Monday night told the nation in a television address that there is no immediate threat to the United States, that the Soviets claim the troop positions are only a training center that won't be enlarged.

The president said he would take a series of steps to monitor the activities of the Soviet forces and to strengthen the U.S. military posture in the area.

"We will monitor the status of the Soviet forces by increased surveillance of Cuba," he said.

Yesterday's SR71 spy flight, the first over Cuba since November 1978 when one was dispatched to determine if Soviet MIG23 aircraft there were equipped to carry nuclear weapons, appears to be a step in the surveillance announced by the president.

Administration sources gave no details about possible future flights.

Carter suspended SR71 overflights of Cuba shortly after taking office in 1977, as a goodwill gesture, but they were resumed temporarily last November in the MIG23 surveillance effort.

Seventeen years ago, in the missile crisis of 1962, Cuba shot down another spy plane, the U2, flying low. The cameras on the SR71 are described as so good that it can spy on huge areas from great altitudes.