(Adds background on U.S. efforts)
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 said in a letter to 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 the letter.
"The force will remain in Chad until its support in resolving the kidnapping situation is no longer required," he said.
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.
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.
(Reporting by Roberta Rampton; Editing by Peter Cooney)