The US is concerned about the health and safety of Niger’s elected president Mohamed Bazoum, who has spent more than two weeks under house arrest.
“We are greatly worried about his health and his personal safety and the personal safety of his family,” a state department spokesman said.
Earlier Mr Bazoum’s party said he and his family were being detained under “cruel” and “inhumane” conditions, Reuters reported.
Mr Bazoum was deposed on 26 July.
Since then, a military junta has ruled Niger while Mr Bazoum has been kept in the presidential palace. Members of the junta have made no comment about the ousted leader’s condition.
On Tuesday US Secretary of State Antony Blinken called Mr Bazoum assuring him of the US’s ongoing support, Washington said.
US State Department spokesman Matthew Miller added: “As time goes on, as he’s held in isolation, it’s a situation that is of growing concern to us.”
Mr Bazoum’s political party, PNDS-Tarayya, claimed in a statement that Mr Bazoum and his family had no access to running water, electricity, fresh goods or doctors.
The statement echoed previous comments made by Niger’s elected prime minister Ouhoumoudou Mahamadou, who has said Mr Bazoum was being held with his wife and son without electricity or water.
The Economic Community of West African States (Ecowas) had given Niger’s coup leaders until Sunday to stand down and restore Mr Bazoum to the presidency.
The head of the presidential guard, Gen Abdourahmane Tchiani, claims he now runs Niger; while the junta appointed ex-finance minister, Ali Mahaman Lamine Zeine, as the new prime minister following the coup.
Ecowas officials are due to meet on Thursday to decide what to do next.
The new military regime has also closed Niger’s airspace until further notice, citing the “threat of military intervention” from Ecowas.
Also on Wednesday, France denied accusations from Niger’s military junta that it is trying to destabilise the country after the coup leaders claimed French aircraft had breached the country’s airspace. Paris said the flight had been authorised by Niger’s military.
Both the US and France operate military bases in Niger as part of operations to disrupt jihadist groups operating in the wider region.
Niger became the main base for French troops after they were told to leave Mali following a coup there.
As part of diplomatic efforts, two envoys of President Bola Tinubu of Nigeria have met the junta in the capital Niamey.