The first appearance of self-proclaimed "caliph" Abu Bakr al-Baghdadi in a video shot in an Iraqi mosque illustrates the extent of his jihadist group's control and confidence, experts say.

Baghdadi, whose Islamic State (IS) group holds territory in both Iraq and Syria, called for Muslims to "obey" him during the prayer sermon at the Al-Nur mosque in Mosul on Friday, according to the video distributed online the following day.

The appearance was surprising for a militant who cultivated an image of a reclusive battlefield commander.

It is the latest in a series of moves that have brought IS back to prominence after it had been on the ropes -- culminating in the offensive it led last month that captured chunks of Iraqi territory.

"Put simply, one of the most wanted men on earth was able to travel into central Mosul and give a 30-minute sermon in the most venerated mosque in the largest city under control of the most notorious jihadist group of our time," said Charles Lister, a visiting fellow at the Brookings Doha Center.

"The fact that Baghdadi has appeared publicly at all in such a central location underlines the extent of confidence felt within his organisation."

The Islamic State spearheaded a Sunni Arab militant offensive that captured Mosul, the second largest city in Iraq, on June 10.

The video posted Saturday showed a portly man clad in a long black robe and turban with a thick greying beard -- purportedly Baghdadi -- addressing worshippers at weekly prayers in central Mosul.

Superimposed text identified the man as "Caliph Ibrahim", the name Baghdadi took when the group on June 29 declared a "caliphate", a pan-Islamic state last seen in Ottoman times, in which the leader is both political and religious.

It marked a remarkable turnaround for IS under Baghdadi's leadership.

When he took over the group, then an Al-Qaeda affiliate and known as the Islamic State of Iraq, it was believed to be reeling from the US military's "surge" of troops and the decision of Sunni tribal militias to turn against it and fight alongside American forces.

- Slowly rebuilt -

But it slowly rebuilt its resources and command structure, later capitalising on the chaos caused by the civil war in neighbouring Syria to expand into the country last year.

Baghdadi subsequently cut all ties to Al-Qaeda, and his influence now rivals that of the group's global chief, Ayman al-Zawahiri.

His group is known across both Iraq and Syria for its brutality, having executed and crucified its opponents in Syria and carried out a litany of bombings across Iraq.

"Everything about the group... has been daring, so it makes sense that Baghdadi would step out of the shadows and into the limelight," said Will McCants, a former counter-terrorism adviser at the US State Department.

"Baghdadi's sermon doesn't make sense from a security perspective but it does make a lot of sense in the context of his competition with Al-Qaeda for leadership of the global jihad," said McCants, now a fellow at the Saban Center for Middle East Policy.

The jihadist group has attracted thousands of foreign fighters to its cause, including many from Western countries, drawn in particular to Baghdadi, who is believed to have joined the insurgency in Iraq following the US-led invasion in 2003 and spent time in an American military prison in the country.

The would-be militants are also attracted, experts say, to the fact that IS is seen as working towards an ideal Islamic emirate and, compared with Al-Qaeda's franchise in Syria, has lower entry barriers.

It has also sought to appeal to non-Arabs by producing magazines and videos in English as well as other European languages.

"This video will likely feature in future... recruitment videos," said Ahmed Ali of the Institute of the Study of War.

"Baghdadi has long sought to position himself as the leader of global jihad in competition with Zawahiri and other figures in the Al-Qaeda central structure. The control of Mosul and other areas in Iraq is the perfect moment for him to establish himself as the main jihadi leader."

"Therefore, a public appearance is important for a 'caliph'."