Cuba put its defense forces on alert last week fearing a possible strike from the United States, reliable Cuban government sources said yesterday.

The decision was made, they said, as a large American fleet appeared to be approaching Cuban waters.

At the same time, the United States began high-level reconnaissance flights over Cuba to determine whether Mig 23 jets here could carry offensive weapons such as bombs.

The situation eased Saturday after Washington repeated announcements that its fleet of 36 warships, including an aircraft carrier and nuclear submarines, was taking part in planned maneuvers.

In the interim Cuba, fearing a possible American strike, had mobilized thousands of troops with tanks, the sources said. Jet troops with tanks, the sources said. Jet interceptors and anti-aircraft defenses, including missile batteries, were also put on alert.

Apparently implicit over the strenous reaction by Washington to the presence of Mig23 planes supplied by the Soviet Union. Cuba was concerned that Washington might be planning a variety ofmoves against it.

The sophisticated M16 jets, first put into operation by the Soviet Union five years ago, are fighters and therefore most suitable for defense.

The United States however, said it was concerned that the planes might have been refitted to permit them to have been refitted to permit them to weapons bombs, making them offensive weapons that might be a violation of the U.S.-Soviet understaning on weapons supplied to Cuba.

Some Western diplomatic observers expressed surprise that Cuba should have been so surprise that Cuba should have been so concerned about the U.S. fleet, which has taking part in maneuvers with Britain.

"An operation like that cannot be mobilized quickly. I'm surprised that Cuba apparently did not know beforehand that an exercise has taking place." a diplomat said.

The 21-day operation had first been announced Nov. 8 by the U.S. Western Atlantic Naval headquarters in Norfolk, Va. It was the largest air-naval exercise in the Gulf of Mexico since the Cuban missile crisis in 1962.

The exercises, which will continue until Dec. 4 are to come no closer than 50 miles to the Cuban coast.