WASHINGTON (Reuters) - The government of Niger has approved the use of armed American drones, a U.S. official said on Thursday, expanding the U.S. ability to target militants in the region. Reuters had already reported that Niger had sought to arm U.S drones against jihadist groups operating on its border with Mali, but it had not been previously reported that an agreement had been reached. An ambush in Niger that killed four U.S. soldiers in October has thrown a spotlight on the U.S. counterterrorism mission in the West African country.     The U.S. official, speaking on the condition of anonymity, said the permission for drones had been granted earlier this week but the capability had not yet been used.  What began as a small U.S. training operation has expanded to an 800-strong force that accompanies the Nigeriens on intelligence gathering and other missions. It includes a $100 million drone base in the central Nigerien city of Agadez which at present only deploys surveillance drones. U.S. forces do not have a direct combat mission in Niger, but their assistance to its military does include intelligence, surveillance and reconnaissance in their efforts to target violent Islamist organizations. 