Press freedom is under grave threat in Niger following the military coup last year that overthrew elected president Mohamed Bazoum, Amnesty International warned on Friday.

"Press freedom in Niger is in jeopardy, as transitional authorities intimidate and arbitrarily arrest journalists," the rights organisation said, adding that since the coup "press freedom has been curtailed and journalists are self censoring amid fear of intimidation and reprisals.

"This disregard for the right to freedom of expression and media freedom and the work of journalists comes at a time when people need accurate information about the conflict affecting them and the transitional authorities' response to it," said Samira Daoud, Amnesty's regional director for West and Central Africa.

"We call on the Nigerien authorities to unconditionally release journalists arrested and detained for exercising their right to freedom of expression or for dealing with sensitive information of public interest, and to ensure that journalists can carry out their work without fear of harassment, intimidation, arrest or detention."

Amnesty highlighted the case of the editor of the L'Enqueteur newspaper, Soumana Maiga, who was arrested on April 24 after his paper published a story about the alleged installation of listening equipment by Russian agents on official state buildings.

Maiga is now in detention, charged with "infringement of national defence" and faces 10 years in jail.

Amnesty also cited the arrest days earlier of Ousmane Toudou, journalist and former communications adviser to Bazoum.

The rights NGO said security forces arrested Toudou three weeks ago after he urged all democrats to oppose the military takeover in a social media post.

Amnesty said Toudou has still not come before a judge, despite the expiry of the legal four-day custody period.

"He is to be tried before a military court in contradiction with international human rights law, which requires military courts to only try military offences," it added.

Niger slid 26 places to 80th of the 180 countries in the annual press freedom ranking published on Friday by Reporters Without Borders.