Soldiers in Niger searched for militants around the airport and military airbase in the capital Niamey on Thursday after explosions and sustained gunfire were heard in what a security source described as an apparent attack on the facility.

The situation appeared mostly calm by noon, with only sporadic fire possibly linked to the army's sweeping operations, a Reuters witness and two residents said, noting that security forces had blocked off the area.

There was no immediate claim of responsibility for the attack. A Niger government spokesperson did not immediately respond to a request for comment.

The airport and military airbase are in the same complex, with the base situated opposite the civilian terminal.

The Islamic State affiliate in the region claimed responsibility for an attack on the complex 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 neighbours 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.

EXPLOSIONS AT DAWN

The first explosions were heard at around 6 am local time on Thursday and sporadic gunfire was still audible nearly two hours later, a Reuters witness said.

Assailants arrived at the airport in two white vehicles, a second security source in Niger said.

A third security source said a group of attackers was hiding in a customs building near the airport.

At least four security officers were killed in the fighting, as well as several of the assailants, a fourth security source said.

On Wednesday, militants launched coordinated attacks on Banibangou and Inates military bases located in the western Tillaberi region, the second and third security sources said.

One of the security sources put the death toll in Banibangou at 10 with more than 40 injured and said that the military had abandoned its base in Inates.

It was not immediately clear how the attacks may have been linked to the explosions and fighting in Niamey.