Businesses and schools were open in the city of about 1.5 million, and people were moving about freely, apart from a cordoned-off zone near the airport which was heavily patrolled by defence and security forces, the Reuters witness said.
Two security sources described the overnight incident to Reuters as a "terrorist attack" and said security had been reinforced around the airport following an internal alert about an imminent attack on the site.
They said a store of uranium currently held at the airport had not been affected by the attack.
Nigerien authorities moved the uranium yellowcake late last year from the Somair mine in Arlit to the Niamey base for export after seizing control of the mine from French nuclear group Orano, according to two other sources, who estimated it to be about 1000 metric tons of uranium.
Two more sources confirmed that the uranium was still at the airport at the time of the incident Wednesday night.
Yellowcake, or uranium oxide concentrate, is a powdered form of uranium which can be processed to make fuel for nuclear power production.
A reinforced security presence was visible on the main thoroughfares of the capital on Thursday, especially on the road leading toward the airport, the witness added.
The heavy gunfire began shortly before midnight on Wednesday and continued for over an hour, the Reuters witness said.
A video shared on social media platform X appeared to show the city's night skyline illuminated by gunfire although Reuters has not independently verified the footage.
A spokesperson for Niger's military government, which seized power in a July 2023 coup, was not immediately available for comment.
The west African country, like its Sahel neighbours Mali and Burkina Faso, has struggled to contain attacks from jihadist groups that have killed thousands and displaced millions in the three countries.
A US missionary was kidnapped in Niamey in October, leading the United States to warn its citizens against travelling to the country.
Five Indian citizens, a Swiss citizen and an Austrian citizen were kidnapped in more remote parts of the country last year.
