"We have shared commercial satellite imagery with the Nigerians and are flying manned ... assets over Nigeria with the government's permission," the official said.
The United States has sent military, law-enforcement and development experts to Nigeria to help search for the missing girls who were kidnapped from a secondary school in Chibok in remote northeastern Nigeria on April 14.
Last week, U.S. Undersecretary for Africa Linda Thomas-Greenfield told Reuters in an interview that Nigeria had requested surveillance and intelligence from the United States. .
Both Nigeria and the United States have said they do not know the whereabouts of the girls, although Nigerian President Goodluck Jonathan has said he believes they are still in Nigeria.