On Friday, the African Union (AU) called on the international community to "save President Bazoum's life". It also urged the military junta to stop its escalation with the bloc, after the Nigerien military refused to allow a joint delegation from the AU and West African states to enter the country for talks.
US Secretary of State Antony Blinken said he had spoken to Mr Bazoum "half a dozen times" since he was imprisoned on July 26 and presidential guard commander Gen Abdourahmane Tchiani declared himself ruler.
"We have deep concern for him, for his family, for his security and well-being," Mr Blinken told reporters.
"We’ve also made clear to the military leaders that we will hold them responsible for his safety and well-being. We talked at some length about the way forward in Niger, and we’re both focused on the same thing, which is the restoration of the constitutional order."
Mr Blinken voiced support for a regional bloc's efforts to reach a "peaceful resolution" to the crisis in Niger. France also expressed its "full support" to the decisions taken by the Economic Community of West African States (Ecowas) on Thursday.
The 15-member bloc ordered the activation of a standby force for possible use against the junta, saying it wanted a peaceful restoration of democracy but all options including military action were on the table.
"I hope that through our collective effort we can bring about a peaceful resolution as a road map to restoring stability and democracy in Niger," Nigeria's President and Ecowas chairman Bola Tinubu said after the meeting.
He said the standby force would be used only as a "last resort".
UN Secretary General Antonio Guterres has expressed concern over the "deplorable" conditions in which Mr Bazoum is being held. Volker Turk, the UN human right chief, described those conditions in comments on Friday.
''I am extremely concerned about the rapidly deteriorating conditions in which President Mohamed Bazoum of Niger, his wife and his son have been arbitrarily detained,'' he said.
''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. I have received reports that electricity has been cut, and they have no access to clean drinking water – amid hot temperatures in Niger – and to necessary medicine.
''Those responsible for the detention of the President must ensure the full respect and protection of his human rights, and of all others being held.''
Ivory Coast President Alassane Ouattara called Mr Bazoum's detention a "terrorist act" and promised his country would supply a battalion of troops to the standby force.