Three U.S. Army Special Forces soldiers were killed and two were wounded in an ambush in Niger on Wednesday while on a routine patrol with Nigerien troops they were training, a U.S. official said.
A spokesman for U.S. Africa Command confirmed the attack after Radio France International (RFI) reported a lethal ambush near the Niger/Mali border.
"We can confirm reports that a joint U.S. and Nigerien patrol came under hostile fire in southwest Niger," said the spokesman.
Namatta Abubacar, an official for the region of Tillaberi in Niger, said five Nigerien soldiers were among the dead.
A Niger diplomatic source said that the attackers had come from Mali and had killed several soldiers.
RFI said earlier on Wednesday a counter-attack was underway.
African security forces backed by Western troops are stepping up efforts to counter jihadist groups forming part of a growing regional insurgency in the poor, sparsely populated deserts of the Sahel.
A relatively new militant group called Islamic State in the Greater Sahara has claimed some of the attacks.
Geoff D. Porter, head of North Africa Risk Consulting, said that any confirmation of Islamic State's role in Wednesday's strike would lead to a strategic shift from Libya towards the Sahel band, stretching eastwards from Senegal to Chad.
"The emphasis ... will now shift south," he said.
The U.S. Africa Command has hundreds of soldiers deployed across the region, including at an air facility in Agadez, and offer training and support to Niger's army in aspects such as intelligence gathering and surveillance.