Military operations were underway on Thursday near Niger's border with Mali the day after three US Army special operations commandos were killed and two others were wounded in an ambush on a patrol in southwest Niger.
Nigerien, US and French troops were conducting military operations in the zone on Thursday, a Nigerien security source said without providing additional details.
An official at Mali's defense ministry said military operations were taking place within Niger and that Malian forces had reinforced their checkpoints along the border.
On Wednesday, the two wounded commandos were taken to Niamey, the capital, and are in stable condition, said US officials, who spoke on condition of anonymity because they were not authorized to discuss the incident publicly.
The officials said the commandos, who were Green Berets, were likely attacked by al-Qaeda in the Islamic Maghreb militants.
In a statement, US Africa Command said the forces were with a joint US and Nigerien patrol north of Niamey, near the Mali border, when they came under hostile fire.
Africa Command said the US forces are in Niger to provide training and security assistance to the Nigerien Armed Forces in their efforts against violent extremists.
The attack marks the first known US combat casualties in Niger.
According to Radio France Internationale, the ambush took place after militants from Mali attacked the village of Tongo Tongo in Tillaberi on Wednesday.
A counter-operation was launched, but the American and Niger soldiers fell into a trap, said the radio report.
The White House said President Donald Trump was notified about the attack Wednesday night as he flew aboard Air Force One from Las Vegas to Washington.
Militant groups form part of a growing regional insurgency in the poor, sparsely populated deserts of West Africa's Sahel.
They have stepped up attacks on UN peacekeepers, Malian soldiers and civilian targets after being driven back in northern Mali by a French-led military intervention in 2013.