The troops, operating the drones from an air base in Bauchi state, are part of a deployment sent by the United States to train their Nigerian counterparts, who are battling jihadist militants.

Washington has also pledged increased intelligence sharing and arms sales to Nigeria, which has been battling an insurgency since 2009.

The use of MQ-9 "Reaper" drones comes after researchers tracked months of US drone flights over Nigeria originating from nearby Ghana last year.

The drones are capable of strikes but are only being used for reconnaissance, said Nigerian Major General Samaila Uba.

"US support is designed to enhance Nigeria's ability to independently detect, track and disrupt terrorist activity," Uba, director of defence information at Nigeria's Defence Headquarters, told AFP.

"This partnership has improved intelligence sharing, operational coordination, and the overall effectiveness of Nigerian-led efforts against violent extremist organizations."

The US troops in Nigeria are not serving in combat roles, both countries have said.

US Africa Command did not immediately respond to a request for comment.

Washington previously operated a major drone base in neighbouring Niger, which has since shuttered after the ruling junta ordered US forces to depart the country.

However, Africom Lieutenant General John Brennan told AFP earlier this year that "we're not in the market to create a drone base anywhere" when asked about replacing the Niger operations.

Late last year, the United States launched strikes targeting militants from the Islamic State Sahel Province in Nigeria's northwest, as local jihadists and those from the neighbouring Sahel region attack areas outside long-standing jihadist strongholds in the northeast.

The strikes had Nigeria's approval but came as Abuja was under pressure from Washington.

US President Donald Trump has criticised Nigeria's government for failing to stop violence in the country, claiming it amounts to the "persecution" of Christians -- a view rejected by many analysts, who say the violence targets members of all faiths.

Nigeria, while pushing back on that framing, has found common ground with Washington in increasing defence cooperation.