Militant groups including ISIS and Al-Qaeda are active across Africa.
The U.S. soldiers were on a training mission with allies from Niger.
The country expects up to 150,000 people to pass through its borders in 2016, many en route to Europe.
The ISIS-affiliated group have launched previously launched attacks in Niger.
Issoufou’s opponent, Hama Amadou, has been imprisoned since November 2015 on baby trafficking charges.
Hama Amadou was imprisoned in November for alleged involvement in a baby trafficking ring.
The guerrillas arrived in the village of Gogone on foot and opened fire indiscriminately on residents, sources said.
The countries will receive support that builds on the training and military equipment the U.S. has already supplied.
Two hundred vehicles transporting troops have entered northeastern Nigeria to take on the Islamist militants.