The EU said on Friday it had "deep concern" about the treatment of Niger's ousted President Mohamed Bazoum, as West African countries prepared for a possible military intervention.
Mr Bazoum and his family have been deprived of food, electricity and health care for several days, said the EU's top diplomat, Josep Borrell.
The President was detained in a military coup on July 26 and the junta has ignored international demands to reinstate him.
The Economic Community of West African States (Ecowas) said on Thursday it was assembling a "standby force" to "restore constitutional order".
The coup in Niger has raised fears of further unrest in a region already under threat from an Islamist insurgency.
German Foreign Minister Annalena Baerbock said on Friday that the military would face "severe consequences" if anything happened to Mr Bazoum, who she said was being "held hostage".
Mr Borrell said the EU was concerned by the "deterioration of the conditions" in which Mr Bazoum and his family were being held.
"We appeal once again for their immediate and unconditional release," he said.
"President Bazoum has dedicated his life to working to improve the daily life of Niger's people. Nothing justifies this kind of treatment."
US Secretary of State Antony Blinken "expressed his grave concern at the continued unlawful detention under deteriorating conditions of President Bazoum and his family."
Mr Blinken "shared that he is particularly dismayed by the refusal of those who seized power in Niger to release Mr Bazoum's family members as a demonstration of goodwill," a State Department statement said.
A statement by Ecowas gave few details on the standby force and left unclear when or whether it would actually be deployed.
It said the bloc was committed to restoring order "through peaceful means", a statement welcomed by western powers.
The French Foreign Ministry expressed its "full support for all of the conclusions adopted" at the emergency Ecowas summit.
France, a former colonial power in the region, said it "reiterates its strong condemnation of the coup attempt under way in Niger and of the detention of President Bazoum and his family".
Ms Baerbock said Germany supported Ecowas's efforts to restore the constitutional order.
"We recognise that countries in the region want to use every diplomatic means possible," she said. "It is in the interests of Niger and the whole region that the coup plotters finally engage with mediation efforts."
US Secretary of State Antony Blinken likewise backed the Ecowas promise to "explore all options for the peaceful resolution of the crisis".
He said the US would hold Niger's military junta "accountable for the safety and security of President Bazoum, his family, and detained members of the government".
Until the coup, Niger was an ally of the West after Mali and others turned against France in favour of closer ties with Russia.
US, French, German and Italian troops are stationed in Niger as part of the fight against the long-running Islamist insurgency in the Sahel region.