Nigeria's President Muhammadu Buhari has ordered the arrest of his predecessor's security adviser who allegedly stole more than $2bn ($1.31).

Sambo Dasuki is accused of awarding phantom contacts to buy 12 helicopters, four fighter jets and ammunition. He strongly denies the allegations.

The equipment was meant to fight militant Islamist group Boko Haram.

Soldiers have complained that despite the military's huge budget, they were ill equipped to fight.

A spokesman for  Buhari said the corruption resulted in the needless deaths of thousands of Nigerians.

Boko Haram has killed thousands in north-eastern Nigeria in its campaign to create an Islamic state.

Buhari took office in May after defeating his predecessor Goodluck Jonathan in elections.

He has vowed to tackle corruption in Nigeria, Africa's most populous state and biggest oil producer.