U.S. Secretary of State Antony Blinken held a telephone call Friday with former Nigerien President Mohamed Issoufou.
Blinken expressed "grave concern" for the continued detention under "deteriorating conditions" of current President Mohamed Bazoum and his family.
"The Secretary shared that he is particularly dismayed by the refusal of those who seized power in Niger to release Bazoum's family members as a demonstration of goodwill," State Department spokesman Matthew Miller wrote in a statement.
"The Secretary assured the former president of the United States' continued dedication to finding a peaceful resolution that ensures Niger can remain a strong partner in security and development in the region," Miller added.
The call came one day after Blinken voiced support for the efforts of the Economic Community of West African States (ECOWAS) bloc following the coup.
"ECOWAS, an organization that brings together West African countries, is playing a lead role in making clear the imperative of the return to constitutional order, and we very much support ECOWAS's leadership and work on this," Blinken told reporters during a news conference with his Mexican counterpart, Alicia Barcena Ibarra.
ECOWAS ordered the activation Thursday of its standby force to "restore constitutional order" in Niger.
Bazoum was ousted July 26 in a military coup led by Gen. Abdourahamane Tchiani, the former commander of Niger's presidential guard, who has declared himself the head of a transitional government.
Bazoum has since reportedly been held hostage at the presidential palace in the national capital, Niamey.
