Karen Bleier | AFP | Getty Images
Demonstrators protest May 14, 2014 in front of the Nigerian Embassy in Washington.
The kidnappings have generated an outpouring of concern from the United States, with Obama's wife, Michelle, speaking outabout the crisis. The president himself has resisted some callsfrom Republicans in Congress to send special forces into Nigeriato search for the girls.
U.S. surveillance aircraft have been flying over remote areas of northeast Nigeria for two weeks, and the Pentagonstruck an agreement last weekend to allow it to shareintelligence directly with the Nigerian government.
The U.S. government has also sent officials from the State Department and the FBI to Nigeria to help in the search.
Two U.S. officials, speaking on condition of anonymity, said the forces were positioned in Chad to allow the surveillanceaircraft more time above the search areas before refueling.
One of the officials said the flights were being carried out by a Predator drone and that the U.S. forces would beresponsible for launch and recovery of the aircraft, as well asforce protection.
The Predator flights were in addition to unmanned surveillance flights already being carried out by Global Hawkaircraft, the official said.
It was not immediately clear how many Global Hawk drones the U.S. military was using to carry out the search.
The composition of America's surveillance aircraft searching for the girls has changed over time and previously includedmanned aircraft as well. The Pentagon said on Tuesday the mannedsurveillance aircraft required maintenance and there were nomanned flights at the moment.