Tweets and online videos are emerging as weapons of war in the Islamic State's campaign to seize a swath of Iraq, with the al-Qaida offshoot's use of social media dwarfing efforts by other militant groups.

Over the weekend, websites previously used by the group calling itself the Islamic State featured a video purporting to show its leader, Abu Bakr al-Baghdadi, delivering a sermon in the captured city of Mosul.

Two weeks ago, an audio recording posted online proclaimed a caliphate across the territory it holds in Iraq and Syria.