(Adds more details on the deployment, background)
WASHINGTON, May 21 (Reuters) - The United States has deployed about 80 military personnel to Chad in its effort to help find and return more than 200 Nigerian schoolgirls kidnapped by Islamist militant group Boko Haram, President Barack Obama told Congress on Wednesday.
"These personnel will support the operation of intelligence, surveillance, and reconnaissance aircraft for missions over northern Nigeria and the surrounding area," Obama said in a letter to congressional leaders.
The forces are to remain in Chad until they are no longer needed, Obama added.
A senior Obama administration official said the military personnel were deployed to Chad with the consent of that government. Some will maintain the unmanned aircraft involved in the mission and the rest will provide security for the group.
The girls were taken in April from a boarding school close to Nigeria's border with Cameroon, Niger and Chad in a sparsely populated region. Their whereabouts are unknown.
The kidnappings have generated an outpouring of concern from the United States, with Obama's wife, Michelle, speaking out about the crisis. The president himself has resisted some calls from Republicans in Congress to send special forces into Nigeria to search for the girls.
U.S. surveillance aircraft have been flying over remote areas of northeast Nigeria for two weeks, and the Pentagon struck an agreement last weekend to allow it to share intelligence 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 surveillance aircraft 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 be responsible for launch and recovery of the aircraft, as well as force protection.
The Predator flights were in addition to unmanned surveillance flights already being carried out by Global Hawk aircraft, 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 included manned aircraft as well. The Pentagon said on Tuesday the manned surveillance aircraft required maintenance and there were no manned flights at the moment.
(Reporting by Roberta Rampton, Phil Stewart and Steve Holland; Editing by Peter Cooney and Andre Grenon)