The Islamic State group Thursday released a propaganda documentary marking a year since it captured Mosul and recounting its surprise at how easily it took over Iraq's second city.

The film glamorises the founding moment of the "caliphate" IS proclaimed less than three weeks later as an epic conquest but it also further documents the collapse of Iraq's security forces.

The 29 minutes of often previously unreleased footage shows the jihadists being welcomed by Mosul residents, prisoners being freed and soldiers desperately attempting to flee in vehicles.

"It was unthinkable that the advance would be so much greater than was planned," said the narrator of the video, which was published on social media on Thursday.

"The operation began with cutting the supply lines of the members of the Safavid army," after which the attack was announced, the narrator says, using a term meaning that Iraqi soldiers are beholden to Iran.

"Three convoys of military vehicles entered the outskirts of the city coming from the Jazeera area," the narrator says, noting that IS forces were heavily outnumbered.

The plan was to "control neighbourhoods on the right side of Mosul to be a starting point for the mujahedeen (holy warriors) to conquer the remainder of the city."

But "the right side of the city of Mosul was conquered and the left side was empty of Safavid soldiers before the men of the Islamic State arrived."

The jihadist group's offensive in Iraq began on June 9. By the following day, IS-led forces had overrun Mosul -- a city of two million -- under their control.

The lightning advance led to the displacement of hundreds of thousands of Iraqis, many of whom fled to the neighbouring autonomous region of Kurdistan.

It also saw a complete collapse of multiple divisions of Iraqi security forces, a dismal performance by an army the United States had spent years and billion of dollars training and equipping.

The IS-led offensive saw the government lose nearly a third of the country, sparking fears the jihadists could even raise their black flag in the capital Baghdad.

Mass mobilisation by mostly Shiite militias and volunteers helped stop the jihadist drive.

The United States and Iran have, separately, led efforts to roll back territorial losses but, one year on, Baghdad is still struggling with a weak army.

IS for its part has survived close to 4,500 air strikes by the US-led coalition and remains a formidable fighting force, thanks in part to the huge arsenal it looted from abandoned government positions in Mosul and elsewhere.