Russian military personnel have entered an air base in Niger that is hosting U.S. troops, a senior U.S. defense official told Reuters, a move that follows a decision by Niger’s junta to expel U.S. forces from the country.

The military officers ruling the West African nation have told the U.S. to withdraw its nearly 1,000 military personnel from the country, which until a coup last year had been a key partner for Washington’s fight against insurgents who have killed thousands of people and displaced millions more.

A senior U.S. defense official, speaking on condition of anonymity, said Russian forces were not mingling with U.S. troops but were using a separate hanger at Airbase 101, which is next to Diori Hamani International Airport in Niamey, Niger’s capital.

The move by Russia’s military puts U.S. and Russian troops in close proximity at a time when the nations’ military and diplomatic rivalry is increasingly acrimonious over the conflict in Ukraine.

It also raises questions about the fate of U.S. installations in the country following a withdrawal.