An Islamic extremist group in Nigeria says it has started kidnapping women and children as part of its bloody guerrilla campaign against the country's government.

In a video released Monday, Boko Haram leader Abubakar Shekau says the group began to kidnap as retaliation for Nigerian security forces taking the wives and children of his group's members. The video shows 12 children, though it does not identify them.

Police and security forces have not discussed any kidnapping cases involving Nigerians taken after Boko Haram attacks, though such abductions could be easily done.

Boko Haram's attacks have been increasing in number and sophistication since 2010. According to an Associated Press count, attacks blamed on the group and other Islamic extremists have killed at least 244 this year alone.