WASHINGTON (Reuters) - The United States has deployed about 80 military personnel to Chad as part of its effort to help find and return more than 200 Nigerian schoolgirls kidnapped byIslamist 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.