Russian military personnel have entered an air base in Niger that is hosting U.S. troops, according to a new report by Reuters.

The move comes following a decision by Niger's junta to expel the nearly 1,000 US military personnel from the country.

Before a coup last year, the US had been a key partner for the Nigerien's fight against insurgents who have killed thousands of people and displaced millions more.

A senior U.S. defence official, speaking on condition of anonymity, said Russian forces were not mingling with U.S. troops but were using a separate hangar 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 nearby at a time when the nation's military and diplomatic rivalry is increasingly acrimonious over the conflict in Ukraine.

"(The situation) is not great but in the short-term manageable," the official said.

Asked about the report, U.S. Defense Secretary Lloyd Austin played down any risk to American troops or the chance that Russian troops might get close to U.S. military hardware.

"The Russians are in a separate compound and don't have access to U.S. forces or access to our equipment," Austin told a press conference in Honolulu.

"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."

Russia is seeking to strengthen relations with African nations, pitching Moscow as a friendly country with no colonial history in the continent.