"Credible reports I have received indicate that the conditions of detention could amount to inhuman and degrading treatment, in violation of international human rights law," UN rights chief Volker Turk said in a statement.

The talks among chiefs of staff of the Economic Community of West African States will take place in the Ghanaian capital Accra, they said on Friday.

AU Commission head Moussa Faki Mahamat said in a statement, joining a chorus of concern about the 63-year-old Nigerien President who was overthrown by military chiefs last month, "Such treatment of a democratically elected president through a regular electoral process is unacceptable."

"Concordant sources attest to a worrying deterioration" of conditions, Faki said.

EU foreign policy chief Josep Borrell and UN Secretary-General Antonio Guterres have also spoken out about the worsening conditions that Bazoum and his family are reportedly living under.

A source close to Bazoum said "he's OK, but the conditions are very difficult," adding that the coup leaders had brandished the threat of assaulting him in the event of military intervention.

Human Rights Watch (HRW) said it had spoken to Bazoum, his doctor, his family lawyer, a former communications adviser and a family friend on Wednesday and Thursday.

Bazoum, 63, described the treatment of him, his wife and their 20-year-old son as "inhuman and cruel", HRW said.

"I haven't had electricity since August 2 and no human contact since August 4. I'm not allowed to receive my family members (or) my friends who have been bringing food and other supplies to us," the group quoted him as saying.

Without power, the family has been forced to eat only dry food, and nothing fresh, Bazoum reportedly said.

"My son is sick, has a serious heart condition, and needs to see a doctor," he was quoted as saying. "They've refused to let him get medical treatment."

President of Nigeria, Bola Ahmed Tinubu (L) with President of ECOWAS Commission Omar Touray (R), during the ECOWAS extraordinary session in Abuja, AFP 

Intervention warning 

Ivory Coast President Alassane Ouattara on Thursday said Niger's military "are holding President Bazoum hostage. I personally think that it's a terrorist act."

Ouattara was speaking on the sidelines of an emergency summit in the Nigerian capital Abuja of the Economic Community of West African States (ECOWAS).

Under pressure to stem a cascade of coups among its members, the bloc approved the deployment of what it called a "standby force to restore constitutional order" in Niger.

Moussa Faki expressed his "strong support" for the decision.

The leaders did not provide any details on the force or any timetable for action, and emphasised they still wanted a peaceful solution.

ECOWAS had previously issued a seven-day ultimatum to the coup leaders to return Bazoum to power.

But the regime defied the deadline, which expired on Sunday without any action being taken.

Troubled region

ECOWAS comprises 15 nations, with Nigeria -- West Africa's most populous nation and economic powerhouse -- the most powerful voice.

Since 1990, the bloc has intervened among six of its members at times of civil war, insurrection or political turmoil.

But the possibility of intervention in deeply fragile Niger has sparked debate within its ranks and warnings from neighbouring Algeria as well as Russia.

ECOWAS members Mali and Burkina Faso, both ruled by military governments that seized power in coups, have warned an intervention would be a "declaration of war" on their countries.

Those two nations, along with Guinea, were not represented at the Abuja summit, nor were Niger's coup leaders.

An adviser to Bazoum who spoke with him recently said the deposed leader was "aware" that an intervention would be "risky".

"He believes that a return to constitutional order is necessary, with or without him" because "the rule of law is more important than his person," the adviser told AFP.