In a letter to House Speaker
The White House previously announced plans to send about 30 military, intelligence and law enforcement advisers to assist the government of Nigeria in its failed, and some say botched, search for the missing girls.
"These personnel will support the operation of intelligence, surveillance, and reconnaissance aircraft for missions over northern Nigeria and the surrounding area," Obama wrote in a notification required under the War Powers Act.
"The force will remain in Chad until its support in resolving the kidnapping situation is no longer required."
Boko Haram, a militant Islamist group that opposes Western education, has taken responsibility for the kidnapping. Its leader has threatened to sell the girls as slaves.
A senior administration official said Wednesday that the deployment to Chad was not a response to new intelligence about the girls' location. There is still no definitive information about their location.
Officials said the team will operate the unarmed surveillance drones from an airbase in Chad, which has given the U.S. temporary basing rights. The aircraft is not normally based there.
National Security Council spokeswoman Caitlin Hayden said the U.S. forces are armed "for their own force protection and security."
Lt Col Myles Caggins, a Pentagon spokesman, said the drone were sent to Chad because to reduce the distance the aircraft must travel to reach the main search area in northern Nigeria, a remote region the size of West Virginia.
"The location on Chad provides the closest proximity to the primary search area," said a U.S military official who asked not to be named discussing military operations.
The U.S. already has flown manned and unmanned surveillance planes over Nigeria from Niger and other locations. But flying the drones from Chad will allow them to search longer before returning to refuel.
Caggins would not say which airfield the drone will use.
Most of the 80 or so Americans are from the Air Force and already have arrived in Chad, Caggins said. Their mission is solely to fly and maintain the drones and to guard the airfield where they will be based, he said.
The decision to base the drones in Chad was made because putting them in Nigeria would have been more of a security risk, he said.