NORFOLK, Va. (AP) _ NATO launched a major exercise Monday in the North Atlantic Ocean with fewer than half the number of U.S. warships that participated in a similar exercise two years ago.

A Navy spokesman said the cutback was not due to the buildup of American Navy forces in the Persian Gulf.

More than 150 ships and 250 aircraft from 11 nations are joining in ''Ocean Safari '87,'' which continues through Sept. 18, the office of Supreme Allied Commander Atlantic said in a statement.

Of those ships, 11 belong to the American Navy, including the aircraft carrier Forrestal and its eight escorts. When the exercise was last held in 1985 with a total of 160 ships, 28 U.S. vessels participated, including three aircraft carriers.

''These exercises are designed a year and a half to two years in advance. It was just designed for a smaller number of U.S. ships this time,'' said Petty Officer 1st Class Jim DeAngio, a SACLANT spokesman. ''More ships from the other nations are involved this time.''

DeAngio said the U.S. cutback had nothing to do with the buildup of naval forces in the Middle East.

The NATO nations hold the exercise to practice defending sea lanes in the Atlantic and general readiness for war.

Sending ships and aircraft to the exercise are Belgium, Canada, West Germany, Italy, The Netherlands, Norway, Portugal, Great Britain, France, Spain and the United States.

This is Spain's first involvement in a major exercise since joining the alliance in 1982. Spain and France are not part of NATO's armed forces, but do join training exercises with other alliance members.

The maneuvers will be conducted in the North Atlantic and in the Norwegian Sea.