WASHINGTON (Reuters) - U.S. forces in Qatar accidentally fired a Patriot missile at a farm in the Gulf Arab state but caused no injuries, the Pentagon said on Tuesday.The U.S. military is investigating the missile launch, first reported by Al Jazeera television. The Patriot is an anti-missile system."Those things are not supposed to accidentally discharge," said Pentagon spokesman Bryan Whitman. "It was not supposed to happen."Whitman said the missile was fired into an unpopulated farm area. The missile was fired during a training exercise, said Lt. Gen. Carter Ham, director of operations for the U.S. Joint Chiefs of Staff.Al Jazeera said the missile was fired from the Assayliyah base.U.S.-allied Qatar was a launch pad for the U.S.-led invasion of Iraq in 2003.