Russia has warned that military intervention in Niger would lead to a "protracted confrontation" after regional bloc Ecowas said it would assemble a standby force.
Such an intervention would destabilise the Sahel region as a whole, the Russian foreign ministry said.
Russia does not formally back the coup. But the US, which backs efforts to restore deposed leader Mohamed Bazoum, says its Wagner mercenary group is taking advantage of the instability.
On Friday coup supporters, some waving Russian flags, protested at a French military base near the capital NIamey, some chanting "down with France, down with Ecowas".
Both France and the US operate military bases in Niger and they have been used to launch operations against jihadist groups in the wider region. Military officials from Ecowas countries are reportedly set to meet on Saturday to draft plans for a military intervention.
The bloc has said it remains open to finding a diplomatic solution to the crisis, but Nigeria's President Bola Tinubu said on Thursday that "No option is taken off the table, including the use of force as a last resort".
The US has not explicitly backed military action but has called on the junta to step aside and allow the restoration of the country's democratic constitution.
The Niger junta has not responded to the latest statements from Ecowas leaders.
Meanwhile fears are growing for the health and safety of Mr Bazoum, who has been held captive since the military seized power on 26 July.
He and his family had been "deprived of food, electricity and medical care for several days", EU foreign policy chief Josep Borrell said.
UN rights commissioner Volker Turk said he had received credible reports that the conditions of detention "could amount to inhuman and degrading treatment".
Rights group Human Rights Watch said Mr Bazoum had told them this week that he and his family were being treated in an "inhuman and cruel" way.
"My son is sick, has a serious heart condition, and needs to see a doctor," HRW quoted Mr Bazoum as telling them.