Military action against coup leaders in Niger would lead to a "protracted confrontation", Russia has warned.
On Friday, the West African bloc Ecowas approved a "standby force" in response to the military takeover in Niger, and Ivory Coast's president said it could be deployed "as soon as possible".
The US has backed efforts to restore Niger's deposed leader Mohamed Bazoum.
Coup supporters, some waving Russian flags, protested at a French military base near the capital.
Moscow has not formally backed the coup but the collapse of Niger's democracy risks pitting Russia against the West in a proxy stand-off.
Both France and the US operate military bases in Niger, which have been used to launch operations against jihadist groups present in the wider region.
Supporters of the military junta have called on Western nations to leave the country and voiced support for Russia.
On Friday protesters were seen chanting "down with France, down with Ecowas" at a French base on the outskirts of the capital city Niamey.
Some were carrying Russian flags, AFP reported, and voiced support for Abdourahamane Tiani, the general now running the country.
In a statement issued on Friday, Russia's foreign ministry warned other West African nations not to send troops into Niger.
It said: "We believe that a military solution to the crisis in Niger could lead to a protracted confrontation in that African country, and to a sharp destabilisation of the situation in the Sahara-Sahel region as a whole."
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: "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.
In a statement, US Secretary of State, Antony Blinken said: "The United States appreciates the determination of Ecowas to explore all options for the peaceful resolution of the crisis."
Separately, Mr Blinken told the BBC earlier this week he believes Russian mercenary group Wagner could seek to take advantage of instability in Niger.
Fears are growing for the health and safety of President Bazoum, who has been held captive since the military seized power on July 26.
United Nations High Commissioner for Human Rights Volker Turk said on Friday: "Credible reports I have received indicate that the conditions of detention could amount to inhuman and degrading treatment, in violation of international human rights law."
European Union foreign policy chief Josep Borrell said he believes Mr Bazoum and his family "have been deprived of food, electricity and medical care for several days".