WASHINGTON (AFP) - In an unprecedented move, the United States on Monday posted up to US$23 million (S$29 million) in rewards to help capture five leaders of militant groups which have spread terror in west Africa and Nigeria.

The highest reward of up to US$7 million is offered for the leader of Nigeria's Boko Haram extremists, Abubakar Shekau, while US$5 million each was posted for Al-Qaeda veteran Mokhtar Belmokhtar and Yahya Abou Al-Hammam, a top figure in Al-Qaeda in the Islamic Maghreb, US officials said.