The video, whose authenticity has not been independently verified, captures Abu Bakr al-Baghdadi -- the self-styled "Caliph" of the self-declared Islamic state established in an area straddling Iraq and Syria -- preaching during Friday prayer at a mosque in Mosul.
"Allah gave me a big responsibility and a noble command," the man purported to be Baghdadi is saying in the video.
"I am not promising wealth and luxury like kings to my subjects, but I am promising what Allah promises to his faithful servants," he says.
On Monday, ISIL announced the establishment of an Islamic state in the region led by Baghdadi.
The move has been widely condemned in the Islamic world, with International Union of Muslim Scholars saying the declaration of the caliphate contravened "Islamic principles and the actual conditions."
Iraq saw a significant increase in sectarian violence between Sunni and Shia Muslims in June, with a coalition of armed opposition groups led by the ISIL taking control of large swathes of the country's predominantly Sunni provinces.
The latest wave of insurgency began when militants captured the city of Mosul on 10 June and soon afterwards took control of a number of other key cities and towns including Tikrit, Tal Afar, Fallujah and Baiji.