The United Nations human rights chief on Friday said there was no legal basis for Niger's military junta to prosecute deposed president Mohamed Bazoum for high treason, saying the "very notion of freedoms" in the country was at stake.

The military junta, which seized power in a coup last month, said it would prosecute Bazoum for high treason over his exchanges with foreign heads of state and international organisations, prompting condemnation from the United States and West African leaders.

"This decision is not only politically motivated against a democratically elected President but has no legal basis as the normal functioning of democratic institutions has been cast aside," UN High Commissioner for Human Rights Volker Turk said in a statement.

"The very notion of freedoms in Niger is at stake," he said. "Generals cannot take it upon themselves to defy - at a whim - the will of the people. Rule-by-gun has no place in today's world."

The coup leaders have imprisoned Bazoum and dissolved the elected government of Niger, a major uranium producer and Western ally in the fight against an Islamist insurgency.

Turk, who called for Bazoum's immediate release, said the Niger coup, the sixth in the region in the past three years, was deeply troubling.