Bazoum, who was elected in 2021, was overthrown on July 26, 2023 by General Abdourahamane Tiani, the head of his presidential guard, over allegations of failing to protect the nation from jihadist attacks.

Since then, he and his wife Hadiza have been held at the presidential palace in Niamey in strict detention.

"The deprivations of liberty of Mohamed Bazoum and Hadiza Bazoum are arbitrary," the UN working group on arbitrary detention said in a report seen by AFP on Monday.

"The appropriate measure would be to immediately free Mr and Mrs Bazoum and to grant them the right to obtain compensation," added the group, which falls under the UN Human Rights Council.

Reiterating a call for their immediate release, a group of lawyers representing Bazoum on Monday said the couple had no contact with the outside world, including family, friends and lawyers, after the confiscation of his phone in October 2023.

"Only a doctor can visit them to bring food and medicine," the collective said in a statement.

"The United Nations has rejected the shaky explanations and confirmed what the world already knows: president Bazoum is being held in a cruel and illegal manner," said Reed Brody, a member of the collective.

Niger's military rulers have told the United Nations that Bazoum had telephone talks with "enemy obscurantist forces to stage an attack with the help of foreign powers" to topple the government.

The UN said the military government had "not furnished any explanation for Bazoum's lengthy detention or for the absence of trials".

In June, a junta-appointed court lifted Bazoum's presidential immunity, paving the way for an eventual trial for which no date has yet been fixed.

In December 2023, the court of the Economic Community of West African States (ECOWAS) had ordered Bazoum's release, but Niger has since left the regional bloc.