WASHINGTON (CNN) -- A U.S. F-15 fighter aircraft on routine patrol over Iraq's northern no-fly zone fired two missiles at a surface-to-air missile site on Monday, government officials told CNN.

The U.S. fighter jets fired at the site after being "illuminated" by Iraqi radar, the officials said.

The F-15 fired once, then circled back and fired again, the sources said.

The U.S. aircraft returned to base without further incident.