Nigeria's army has offered £1 million for information leading to the capture of 19 leading members of Islamist sect Boko Haram, which has killed hundreds this year.

Boko Haram wants to impose strict sharia – or Islamic – law in the country of 160 million, split equally between Christians and Muslims, and the group has become the biggest security threat to Africa's largest oil producer.

The military joint task force in northeast Borno state, the focus of the attacks, offered £200,000 for the sect's leader, Abubakar Shekau.