Joint military drills between South Korea and the U.S. have begun, amid rare family reunions between North and South Korean relatives separated for decades.The annual military exercises will last until 18 April.They will involve Key Resolve, a computer-based simulation, and Foal Eagle, which involves air, ground and naval drills.Pyongyang is opposed to the drills and had previously threatened to cancel the reunions if the exercises went ahead.The military drills will involve more than 12,500 U.S. troops.The U.S. and South Korea describe the annual drills as defensive in nature, but Pyongyang has described them as "exercises of war".