The absence of such evidence comes as the administration of President Barack Obama debates how Washington and its allies can best support Nigeria and its neighbours. Some U.S. lawmakers already argue that U.S. aid to the region has been too heavily weighted towards security.

DEBATE ON ASSISTANCE

U.S. security assistance to the four African countries plagued by Boko Haram – Nigeria, Niger, Chad and Cameroon – has soared to more than $400 million since 2014, surpassing aid for governance, human rights, education and rebuilding infrastructure, according to a recent Congressional Research Service report.

The Obama administration is poised to approve the sale of 12 attack aircraft to Nigeria, Reuters reported last month.

The United States also has offered to send a Special Operations mission to advise Nigerian units, and has dedicated more intelligence and surveillance assets to help African forces fight Boko Haram.

Still, some U.S. government experts warn that defeating it requires Nigeria to boost policing, education and development in its Muslim-dominated northeast and to crack down on corruption.

Administration officials say that it’s easier to win congressional support for military assistance to fight extremist groups – especially if defence contracts are involved – than it is to muster backing for steps to attack radicalism at its roots.

While it is estimated to have killed more than 15,000 people since 2009, Boko Haram has not attacked U.S. interests and has deep roots in Nigeria’s Christian-Muslim divide, which long predates the Syrian-based Islamic extremist group.

Those uncertainties have fuelled tension over how best to combat the group, and even how to characterise it. In public, U.S. officials rarely call the group Islamic State-West Africa Province, the name it adopted in March 2015.