The US military withdrew its personnel from Niger's Air Base 101 near the airport in the capital Niamey on Sunday, ahead of its exit from a major drone base near the desert city of Agadez in the coming weeks.

Niger's ruling junta in April ordered the U.S. to withdraw its nearly 1 000 military personnel from the country following a coup last year in the West African nation.

"Thanks to effective cooperation and communication between the Nigerian and American armed forces, this operation was completed ahead of schedule and without any complications," Niger's defence ministry and the US Department of Defense said in a joint statement.

The focus will next shift to withdrawal from the $100-million drone base near Agadez in central Niger, which had provided crucial intelligence about jihadist-linked groups, US Air Force Major General Kenneth Ekman said on Friday.

The pullout from that base, known as Air Base 201, will likely take place in August, he said.

Niger's ruling junta has given the US until September 15 to remove troops from its territory.

In April, Russia sent military trainers to Niger.

Niger and its military-led neighbours Mali and Burkina Faso signed a confederation treaty on Saturday, signalling an ever-closer alignment between the countries in the insurgency-torn central Sahel.

They have torn up defence agreements with US, European and United Nations forces.