The United States has deployed a drone and 80 troops to Chad to help with regional efforts to rescue more than 200 schoolgirls kidnapped in neighboring Nigeria, officials said Wednesday.

"These personnel will support the operation of intelligence, surveillance and reconnaissance aircraft for missions over northern Nigeria and the surrounding area," President Barack Obama said in a letter to Congress.

"The force will remain in Chad until its support in resolving the kidnapping situation is no longer required," he said, without mentioning the type of aircraft deployed.

A US defense official, who spoke on condition of anonymity, told AFP the troops will oversee at least one unarmed surveillance drone to try to track those who kidnapped the Nigerian schoolgirls.

Last month, Islamist militants from the Boko Haram extremist group seized more than 200 Nigerian girls from school dormitories in the northeastern community of Chibok, near border with Chad.

Nigerian and international authorities were slow to react but, after concerned citizens launched social media campaign and the group threatened to sell the girls into slavery, offers of help have multiplied.

France also has forces in Chad, and Britain and the United States have sent small teams of specialists to Nigeria to assist President Goodluck Jonathan's government in the search for the girls.

Washington recently began surveillance drone flights over northeastern Nigeria to try to track down the location of the kidnapped girls.

Meanwhile, the violence has continued, with attacks on villages attributed to Boko Haram leaving scores dead and a bomb attack on the Nigerian city of Jos killing at least 118 people Tuesday.