WASHINGTON (CNN) -- A U.S. fighter aircraft fired a missile at an Iraqi ground-radar installation early Tuesday morning near Mosul in northern Iraq's no-fly zone, the National Security Council reported Tuesday. It was the second such incident in two days.

A Defense Department duty officer provided no further information on the incident. The U.S. pilot returned to base safely.

U.S. pilots patrolling the northern and southern no-fly zones in Iraq are authorized to fire when they feel threatened. If a pilot finds that enemy radar has targeted his or her aircraft, the pilot is authorized to fire.

There have been a number of incidents when pilots were "painted" by enemy radar or were fired on by anti-aircraft fire in the past two weeks.

U.S. Defense Secretary William Cohen has warned Iraq that the United States will continue to patrol the no-fly zones and will retaliate in cases of provocation.

The northern no-fly zone has been patrolled by U.S. and British pilots since April 1991 to protect Kurdish rebels.