ABUJA - US surveillance aircraft were flying over remote areas of northeast Nigeria as part of an international hunt for about 200 girls kidnapped a month ago by Boko Haram militants who stormed their school.
Thousands of Nigerian troops have been sent to the region, while the United States and Britain also have teams on the ground to help with the search.
The mass abduction of the girls from their boarding school in Chibok has caused international outrage and Nigerian President Goodluck Jonathan's government has faced criticism from parents and others for its a slow response.
It has also brought global publicity to Boko Haram, which has killed killed thousands of Nigerians since it took up arms in 2009 to fight for an Islamist state.
The group had initially threatened to sell the girls into slavery but on Monday offered to trade them for detained militants.
The US State Department said Washington had sent military, law-enforcement and development experts to Nigeria to help search for the missing girls.
"We have shared commercial satellite imagery with the Nigerians and are flying manned ISR (intelligence, surveillance and reconnaissance) assets over Nigeria with the government's permission," a U.S. official said.