A leader of the Nigerian militant group Boko Haram has promised more attacks in a new video posted on the Internet.

The 15-minute video, seen on YouTube, shows Imam Abubakar Shekau, who is believed to head a faction of Boko Haram.

In the video, Shekau says his group's fighters will continue to kill and are ready and willing to be killed themselves as martyrs.

Nigerian authorities blame Boko Haram for hundreds of deaths in bombings and shootings over the past 18 months. The group has claimed responsibility for several of the attacks, including a Christmas Day bombing of a church near Abuja that killed more than 30 people.

Shekau says Nigerian President Goodluck Jonathan, who has vowed to crush the group, is taking on a battle he cannot win. Shekau describes Nigeria's constitution and democracy as “sacrilege.” He says Boko Haram is ready for peace talks but only on terms that are in conformity with its beliefs.

The group is believed to want wider implementation of sharia, or Islamic law, across Nigeria. It recently warned Christians in Muslim-majority northern Nigeria to leave the area.

In response to the violence, President Jonathan recently declared a state of emergency in 15 locations.

The president has also deployed extra troops to the north, but attacks have continued.

Late Tuesday, suspected Boko Haram gunmen shot and killed eight people, including four police officers, in the town of Potiskum in Yobe state.

Also Tuesday, an angry mob attacked a mosque and a school in the southern city of Benin. The Nigerian Red Cross said five people were killed, and an official said the attacks were linked to tensions between Muslims and Christians.