ABUJA: Nigeria's president has ordered an investigation into the killing of the leader of an radical Islamist sect behind nearly a week of violence that killed over 700 people.

President Umaru Yar'Adua said he has also ordered the national security adviser to arrange a post-mortem on Boko Haram leader Mohammed Yusuf.

Yusuf was killed on July 30 after he was found hiding in a relative's goat pen. Police said Yusuf was killed in a gunfight but that has been disputed by a top officer who said he personally arrested Yusuf and turned him over alive to authorities.

Yar'Adua said Tuesday that Yusuf's death was a serious issue and he hopes to get the post-mortem report by the end of the week.

"This is an incident that will be investigated, together with the overall events that have happened," Yar'Adu said.

"I directed the national security adviser to carry out a post-mortem with the security agencies as a first step so that we can have a full report of what has happened during the crisis including how the leader of the Boko Haram was killed, and the circumstances under which he was killed," he said.

Moderate Muslim clerics and scholars said they had warned government officials about the sect's violent tendencies and those alarms went unheeded before Boko Haram militants attacked a police station in Bauchi state on July 26. Violence quickly spread to three other states before Nigerian forces retaliated, storming the group's Maiduguri compound.

Boko Haram — translated as "Western education is sacrilege" seeks the imposition of strict Islamic Shariah law in Nigeria, a multi-religious country that is a major oil producer and Africa's most populous nation.

The 39-year-old Yusuf had managed to escape on July 29 along with some 300 followers as troops shelled his compound in the city of Maiduguri, killing at least 100 people, including Yusuf's deputy.

Police, who invited local journalists to view Yusuf's battered corpse on July 30, insisted he was fatally wounded in combat. But Army Col. Ben Ahanotu said he personally arrested Yusuf and handed him over alive to police.

A video obtained by AP Television News from police shows what authorities say is Yusuf's body. The corpse is in the middle of a street and the victim's hands are cuffed. The injuries were severe, with gaping wounds to sections of his arms and abdomen. Bursts of gunfire can be heard in the background.

Human Rights Watch and Amnesty International also called for investigations into Yusuf's death and other killings during the upheaval in predominantly Muslim northern Nigeria.