The Russian presence on the base comes as tensions remain high between Washington and Moscow over the ongoing US support for Ukraine's military.
Russia has moved some troops onto an airbase in Niger where a small number of US forces remain, but Defence Secretary Lloyd Austin said he doesn't see it as a significant issue. According to a US official, most American troops previously stationed at that base in Niamey have departed.
The arrival of Russian trainers in the West African country about three weeks ago came in the wake of Niger’s decision to order out all US troops. The order has impacted US military operations in the Sahel, a large region south of the Sahara Desert where groups linked to al-Qaida and the Islamic State group are active.
The Pentagon has said the US troops will depart but has not provided a timeline.
When Russian troops arrived last month, it was unclear where they were staying. The Niamey base, Austin said late on Thursday, is located at the capital city's Diori Hamani International Airport, and “the Russians are in a separate compound and don’t have access to US forces or access to their equipment”.
He said the US will continue to watch the situation but he doesn't see it as a significant force protection issue.
A US official said the Russian forces are on the other side of the Niamey facility, known as Airbase 101, and that other international forces - such as the Germans and Italians - also reside. The official spoke on the condition of anonymity to discuss troop movements.
It's unclear how many US troops remain at the Niamey base.
The Russian presence on the base comes as tensions remain high between Washington and Moscow over the ongoing US support for Ukraine's military.
About 1,000 US troops are still in Niger, but the bulk of them moved to what's called Airbase 201 near Agadez, some 920 kilometres away from the capital, not long after mutinous soldiers ousted the country’s democratically elected president last July.
A few months later, the junta asked French forces to leave and turned to the Russian mercenary group Wagner for security assistance.
In October, Washington officially designated the military takeover as a coup, which triggered US laws restricting the military support and aid that it can provide to Niger. Since then, diplomatic efforts to restore ties with Niger have been unsuccessful.
Until recently, Washington considered Niger a key partner and ally in a region swept by coups in recent years, investing millions of dollars in the Agadez base, which has been critical to US counterterrorism operations in the Sahel.
The US also has invested hundreds of millions of dollars in training Niger’s military since it began operations there in 2013.
The Pentagon also has said the US will relocate most of the approximately 100 forces it has deployed in neighbouring Chad for now. Chad is also considering whether to continue its security agreement with the US.