EU chief Charles Michel warned there will be "grave consequences" if Niger's military regime allows ousted President Mohamed Bazoum's health to worsen under house arrest, a European official said Friday.

In a call to President Bola Tinubu of Nigeria, chair of the ECOWAS regional bloc which opposes the Niger coup, Michel said "President Bazoum's detention conditions are deteriorating".

Bazoum, 63, was detained on July 26 by members of his presidential guard, in the fifth coup to hit Niger since independence from France in 1960. He and his family are detained in the presidential palace.

"The Nigerian president stressed the determination and political will of ECOWAS to act together. ECOWAS will maintain their sanctions despite the economic impact these have on some of the countries from the region," the official said.

Michel "reiterated the EU's full support and backing of ECOWAS' decisions, as well as firm condemnation of the unacceptable coup de force in Niger.

"The EU will not recognise the authorities resulting from the putsch in Niger. President Bazoum, democratically elected, remains the legitimate head of state of Niger," she said.

"President Bazoum's detention conditions are deteriorating. Any further deterioration to his well-being status will have grave consequences."