Russian military personnel have entered an air base in Niger that is hosting US troops, a senior US defence official told Reuters, a move that follows a decision by Niger's junta to expel US forces.

The military officers ruling the West African nation have told the US 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 US defence official, speaking on condition of anonymity, said Russian forces were not mingling with US 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 US 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 US 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, US Defense Secretary Lloyd Austin played down any risk to American troops or the chance that Russian troops might get close to US military hardware.

"The Russians are in a separate compound and don't have access to US 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 US 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, US 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 in 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 US military and financial aid for Ukraine in its effort to defend against invading Russian forces.

The US 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 US 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 US 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 Islamic State and al Qaeda affiliate Jama'at Nusrat al-Islam wal Muslimeen (JNIM) fighters with armed drones.

Washington is concerned about Islamic militants in the Sahel region, who may be able to expand without the presence of US forces and intelligence capabilities.

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

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

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

A two-star US 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 US troops in Niger, the official said the plan was for them to return to US Africa Command's home bases, located in Germany.