The UN criticised Nigerien generals who led a coup to seize power in the country “on a whim”, as the West African regional bloc, Ecowas, is expected to make a decision on military intervention to restore the democratically elected government.
“The very notion of freedom in Niger is at stake,” UN rights chief Volker Turk said on Friday.
“Generals cannot take it upon themselves to defy – at a whim – the will of the people,” he said. “Rule by gun has no place in today's world.”
Violence erupted this week in a string of villages in remote southwest Niger leaving at least 28 civilians dead, sources told AFP on Friday.
"For now we have recorded at least 28 dead, but the toll could go up," a senior official in the Tillaberi region close to Mali, told AFP.
Some of the bodies had been "carried off" by the Niger river, he added.
Last month, a coup led by Gen Abdourahmane Tchiani toppled Niger's President Mohamed Bazoum. He has been held by the junta since then, and is being threatened with charges of treason.
West Africa military chiefs began their Niger crisis talks on Thursday in Ghana.
The use of force remains a last resort but “if everything else fails, the valiant forces of West Africa … are ready to answer to the call of duty”, Ecowas Commissioner for Political Affairs, Peace and Security Abdel-Fatau Musah said at the start of the event on Thursday.
Speaking to the Associated Press, a high-ranking member of Mr Bazoum's political party, Boubacar Sabo, said he had been kidnapped by members of the presidential guard who overthrew him on July 26 and have since kept him under house arrest.
“What is happening in Niger, if it succeeds, is the end of democracy in Africa. It’s over. If we fight today, it is to prevent these kinds of things from happening and to ensure a future for our continent,” Mr Sabo said.
Details of a potential Niger operation have not been released and analysts say any intervention would be politically and militarily risky, especially for regional player Nigeria.
Nigeria is already struggling to contain violence from several armed groups at home, and leaders in the country's north have warned about spillover from Niger across the border if there is an intervention.
Ecowas troops have intervened in other emergencies since 1990, including civil wars in Liberia and Sierra Leone. The Ivory Coast, Benin and Nigeria are expected to contribute troops to a Niger mission.
Niger's coup leaders have warned against any military strikes, but they have also said they are open to talks.
The military-ruled governments in neighbouring Mali and Burkina Faso have also said an intervention in Niger would be seen as a declaration of war against them.
Russia and the US have called for a diplomatic solution to the crisis.
Ecowas has already applied trade and financial sanctions on Niger while France, Germany and the US have suspended aid programmes.
Germany has also said it wants the EU to impose sanctions on the coup leaders, saying that Foreign Minister Annalena Baerbock had held talks with her French and US counterparts.