The United States (US) has reportedly been conducting intelligence-gathering flights over large parts of Nigeria since late November this year.
According to flight-tracking data, current and former US officials, as reported by Reuters, the development signalled increased security cooperation between the two countries.
However, the report could not determine the specific information the flights were gathering.
The surveillance missions in West Africa follow threats made in November by President Donald Trump to militarily intervene in Nigeria over what he described as the government's failure to stop violence targeting Christian communities.
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The flights are also taking place just months after a US pilot working for a missionary agency was kidnapped in neighbouring Niger Republic.
According to the Reuters' report, the US contractor-operated aircraft used for the surveillance missions typically take off from Ghana, fly over Nigeria and then return to Accra, the Ghanaian capital, as shown by flight-tracking data for December.
The data indicate that the operator is Mississippi-based Tenax Aerospace, a company that provides special-mission aircraft and works closely with the US military, according to its website. Tenax Aerospace did not respond to requests for comment.
Liam Karr, Africa team lead for the Critical Threats Project at the American Enterprise Institute, analysed the flight data and said the operation appeared to be departing from an airport in Accra, a known hub for the US military's logistics network in Africa.
Karr said the operation was an early sign the US was rebuilding its intelligence and surveillance capacity in the region after Niger ordered US troops to leave a sprawling, newly built air base in the desert last year and instead turned to Russia for security assistance.
"In recent weeks, we have seen a resumption of intelligence and surveillance flights in Nigeria," Karr said in an interview.
A former US official said the aircraft was among several assets the Trump administration moved to Ghana in November.
While it remains unclear how many aircraft are still based there, the former official said the missions include efforts to track the kidnapped US pilot and gather intelligence on militant groups operating in Nigeria.
Boko Haram and its splinter group, the Islamic State West Africa Province, are among the militant organisations active in the country.
