The US has deployed about 80 military personnel to Chad in its effort to help find more than 200 Nigerian schoolgirls kidnapped by Islamist militant group Boko Haram.
In a letter to Congress, US President Barack Obama said the personnel will support "the operation of intelligence, surveillance, and reconnaissance aircraft for missions over northern Nigeria" and its surroundings.
"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.
US 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 US government has also sent officials from the State Department and the FBI to Nigeria to help in the search.
Meanwhile, suspected Nigerian Islamist militants killed 17 people in a remote northeastern village last night, hours after a bomb killed 118 people in the central city of Jos, police have said.
Militants opened fire on Alagarno village and razed several houses to the ground, a source at police headquarters told Reuters.
The attack was barely 30km from Chibok, where Boko Haram Islamists abducted the schoolgirls last month.
Separately, police said men on motorbikes had killed nine people in a raid on the nearby village of Shawa on Monday.
There was no immediate claim of responsibility for either assault, but Boko Haram has either claimed or been blamed for scores of similar attacks in that part of Borno state, near the hilly border with Cameroon.
In the past two months, the group has redoubled its five-year-old violent campaign to carve an Islamic state out of religiously-mixed Nigeria.
Bomb attacks are growing more frequent and sophisticated, including two on the capital last month, and massacres of helpless villagers are an almost daily occurrence.