Russian military personnel have reportedly entered an air base in Niger where U.S. troops are stationed, according to a senior U.S. defense official, following Niger's junta's decision to expel American forces.

The ruling military officers in the West African nation have instructed the U.S. to remove its nearly 1,000 military personnel from the country. Until a coup last year, Niger had been a crucial partner for Washington in its efforts against insurgents, who have caused widespread casualties and displacement.

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 hangar at Airbase 101, which is next to Diori Hamani International Airport in Niamey, Niger's capital.

The move by Russia's military, which Reuters was the first to report, 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.

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

Asked about the Reuters 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."

The Nigerien and Russian embassies in Washington did not immediately respond to a request for comment.

The U.S. and its allies have been forced to move troops out of a number of African countries following coups that brought to power groups eager to distance themselves from Western governments. In addition to the impending departure from Niger, U.S. troops have also left Chad in recent days, while French forces have been kicked out of Mali and Burkina Faso.

At the same time, Russia is seeking to strengthen relations with African nations, pitching Moscow as a friendly country with no colonial baggage on the continent.

Mali, for example, has in recent years become one of Russia's closest African allies, with the Wagner Group mercenary force deploying there to fight jihadist insurgents.

Russia has described relations with the United States as "below zero" because of U.S. military and financial aid for Ukraine in its effort to defend against invading Russian forces.

The U.S. official said Nigerien authorities had told President Joe Biden's administration that about 60 Russian military personnel would be in Niger, but the official could not verify that number.

After the coup, the U.S. military moved some of its forces in Niger from Airbase 101 to Airbase 201 in the city of Agadez. It was not immediately clear what U.S. military equipment remained at Airbase 101.

The United States built Airbase 201 in central Niger at a cost of more than $100 million. Since 2018 it has been used to target Daesh and al Qaeda affiliate Jama'at Nusrat al-Islam wal Muslimeen (JNIM) fighters with armed drones.

Washington is concerned about terrorists in the Sahel region, who may be able to expand without the presence of U.S. forces and intelligence capabilities.

Niger's move to ask for the removal of U.S. troops came after a meeting in Niamey in mid-March, when senior U.S. officials raised concerns including the expected arrival of Russian forces and reports of Iran seeking raw materials in the country, including uranium.

While the U.S. message to Nigerien officials was not an ultimatum, the official said, it was made clear U.S. forces could not be on a base with Russian forces.

"They did not take that well," the official said.

A two-star U.S. general has been sent to Niger to try and arrange a professional and responsible withdrawal.

While no decisions have been taken on the future of U.S. troops in Niger, the official said the plan was for them to return to U.S. Africa Command's home bases, located in Germany.