LAGOS, Nigeria — The leader of a radical Islamist sect has challenged the authority of Nigeria's president in an online video, promising more attacks in a nation increasingly overcome by unrest and divided by religion.

The video of Imam Abubakar Shekau cements his leadership in the sect known as Boko Haram. Analysts and diplomats say the sect has fractured over time, with a splinter group responsible for the majority of the assassinations and bombings carried out in its name.

It also exploits the widening mistrust those living in Nigeria's Muslim north feel for a weak federal government run by a Christian president, who has sparked a nationwide strike and protests after removing subsidies that kept gasoline prices low.

Boko Haram, whose name in Hausa means "Western education is sacrilege," has carried out attacks in Nigeria's northeast and its capital that killed at least 510 people last year alone, according to an Associated Press count. The sect is blamed by the government for killing at least 68 people in the last week alone, as it continues its campaign to impose strict Islamic Shariah law across the multiethnic nation of more than 160 million people.

On Wednesday, suspected Boko Haram gunmen attacked a bus in Yobe state carrying Christian Igbo traders fleeing the north for their homeland in the country's southeast, killing four people, police said. In Adamawa state, authorities said sect members shot to death a police officer.