Explosions and sustained gunfire were heard early on Thursday morning at the airport and military airbase in Niger's capital Niamey, witnesses said, in what a security source described as an apparent attack on the facility.
There was no immediate claim of responsibility for an attack. A Nigerien government spokesperson did not immediately respond to a request for comment.
The Islamic State-affiliate in the region claimed responsibility for an attack on the airport in January. At the time, the group said it had targeted air command headquarters and drone assets and claimed to have "delivered a direct blow" to the Sahel country's counterinsurgency operations.
Niger, like its Sahel neighbors Mali and Burkina Faso, has struggled to contain attacks from jihadist groups linked to al Qaeda and Islamic State that have killed thousands and displaced millions across the three countries.
On Thursday morning, the first explosions occurred at around 6 a.m. local time (0500 GMT) and sporadic gunfire was still audible nearly two hours later, the Reuters witness said.
Security forces had blocked off the area, the witnesses said.
In the attack in January, Niger's Defense Ministry said militants had arrived on motorcycles and security forces quickly repelled them. It said four soldiers were wounded.
Material losses in that attack included a cache of ammunition that caught fire, the ministry said at the time, adding that there was damage to several civilian airplanes.
Niger's military ruler, Abdourahamane Tiani, accused the presidents of France, Benin, and Ivory Coast of sponsoring the January attack, without offering any evidence. He also vowed retaliation.
This is a developing story.