The United States will hold the junta that took power in Niger accountable for the safety of democratically elected President Mohamed Bazoum, his family, and detained members of the government, US Secretary of State Antony Blinken said on Thursday.

"The United States joins the Economic Community of West African States (ECOWAS) in calling for the restoration of constitutional order in Niger," Blinken said in a statement released by the US State Department.

The United States recently paused certain foreign assistance programmes for Niger after the military junta overthrew the democratically elected president.

West Africa's regional bloc on Thursday ordered the activation of a standby force for possible use against the junta that took power in Niger in July, saying it wanted a peaceful restoration of democracy but all options including force were on the table.

"The United States appreciates the determination of ECOWAS to explore all options for the peaceful resolution of the crisis," Blinken said.

The threat of an invasion, though not specific, raises tensions in and around Niger, a uranium producer that until the coup was an important ally of the West in the fight against Islamist insurgents devastating the Sahel region.

The junta, which seized power on July 26, had defied an August 6 deadline to stand down set by ECOWAS, instead closing Niger's airspace and vowing to defend the country against any foreign attack.

The bloc pledged to enforce sanctions, travel bans and asset freezes on those preventing the return to power of Bazoum.