Niger's capital Niamey was calm on Wednesday morning following heavy gunfire near the presidential palace during the night after a unit from a nearby air base tried to seize the palace, a senior Niger security source said.

The assailants were pushed back by heavy shelling and gunfire from the presidential guard unit, said three other security sources who requested anonymity because they are not authorised to speak to media.

The sources did not say where the assailants had gone or comment on the whereabouts of President Mahamane Ousmane or president-elect Mohamed Bazoum, who is due to be sworn in on Friday after an election victory disputed by Ousmane.

Former US Sahel envoy J. Peter Pham tweeted that both men were safe. The government of Niger was not immediately available to comment.

Bazoum's election is the first democratic transition of power in the west African state that has witnessed four military coups since independence from France in 1960.

The heavy gunfire started around 3 am local time (0200 GMT) and lasted for around 30 minutes, according to a Reuters witness. By 10 am roads were open and the situation appeared normal, said several witnesses.

There have been growing attacks by Islamist militants, and protests in the country following Bazoum's victory in a February presidential election runoff. Former president Ousmane, who lost that contest, has rejected the results and said there was fraud.

The US Embassy in Niamey said in a statement it was closed for the day due to gunshots heard in the neighbourhood and warned that the security situation remained fluid in the post-election period.