Russian troops have been deployed to an airbase in Niger where American soldiers are located, US officials say.
The move comes after Niger's military rulers ordered the US to withdraw troops who had been countering Islamic insurgents in the region.
Russian forces were present at the base in Niger's capital Niamey, but were not mingling with American troops, US officials told the Reuters news agency.
Defence Secretary Lloyd Austin said the Russians posed no "risk" to US forces.
"The Russians are in a separate compound and don't have access to US forces or access to our equipment," Mr Austin told journalists in Honolulu, Hawaii.
"I'm always focused on the safety and protection of our troops... But right now, I don't see a significant issue here in terms of our force protection," the defence secretary added.
Niger is in Africa's Sahel region, which is considered the new global epicentre of the Islamic State group.
The US has relied on the country as its primary base for monitoring regional jihadist activity, but its relations with the country's ruling military regime have deteriorated since it condemned last year's coup.
In turn, Niger has turned to Russia for assistance fighting Islamist insurgents in the south of the country.
