The first explosions occurred at around 6 am local time (0500 GMT) and sporadic gunfire was still audible nearly two hours later, the Reuters witness said.
A security source said it appeared the airport was under attack.
A Niger government spokesperson did not immediately respond to a request for comment.
The Daesh (ISIL or ISIS) terrorist group’s affiliate in the region claimed responsibility for an attack on the airport in January.
Niger, like its Sahel neighbors Mali and Burkina Faso, has struggled to contain attacks from terrorist groups linked to al Qaeda and Daesh that have killed thousands and displaced millions in the three countries.