Self-proclaimed caliph Abu Bakr al-Baghdadi made an unprecedented appearance in the Iraqi city of Mosul, which his forces helped capture last month, and ordered Muslims to obey him, according to a video posted Saturday.

That marks a significant change for the shadowy jihadist whose Islamic State (IS) group led a lightning offensive that overran swathes of five provinces north and west of Baghdad.

The onslaught has alarmed world leaders, displaced hundreds of thousands and piled pressure on Prime Minister Nuri al-Maliki as he seeks a third term in office following April elections.

The video showed a portly man clad in a long black robe and a black turban with a long greying beard addressing worshippers at weekly prayers at Al-Nur mosque in central Mosul.

"I am the wali (leader) who presides over you, though I am not the best of you. So if you see that I am right, assist me," said the man, purportedly Baghdadi.

"If you see that I am wrong, advise me and put me on the right track, and obey me as long as I obey God."

- Pan-Islamic 'caliphate' -

Text superimposed on the video 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.

The video is the first ever official appearance by Baghdadi, says Aymenn al-Tamimi, an expert on Islamist movements, though the jihadist leader may have appeared in a 2008 video under a different name.

Baghdadi is believed to have been born in the Iraqi city of Samarra in 1971, and joined the insurgency against the US military following the 2003 invasion that ousted dictator Saddam Hussein.

He spent time in a US military prison and eventually took over leadership of a group, then affiliated with Al-Qaeda and known as the Islamic State of Iraq, in 2010.

At the time, the group was believed to be on the ropes, but Baghdadi led it back to prominence.

Last year, the organisation expanded into Syria, becoming a major player in the war to oust President Bashar al-Assad.

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

IS is known for its brutality, executing and crucifying opponents, and photographs emerged Saturday showing its militants demolishing Sunni and Shiite mosques and shrines in Mosul and surrounding Nineveh province.

Iraqi security forces wilted when faced with the initial IS-led onslaught, and while they have since performed more capably, they have struggled to retake territory from insurgents.

An assault on Saddam's hometown of Tikrit has gone on for more than a week without retaking the city, while a suicide car bomb killed 15 people Friday near the sensitive shrine city of Samarra.

Iraq has reached out for international assistance and Washington has sent military advisers, but Baghdad's request for American air strikes against the militants has been rebuffed.

Tehran has also pledged assistance, and state media reported Saturday that an Iranian pilot was killed in Iraq, without providing details on whether he died while flying sorties or fighting on the ground.

Maliki's security spokesman, meanwhile, told AFP Saturday that ground forces commander Ali Ghaidan and federal police chief Mohsen al-Kaabi have both been sacked, the most senior officers to be dismissed since the militant offensive began.

- Government formation deadlock -

The crisis has polarised Iraq's Shiite Arab, Sunni Arab and Kurdish communities, just as their leaders look to form a new government following April elections.

Maliki, labelled at home and abroad as authoritarian and sectarian, insisted Friday that he would not give up on his quest for a third term. He pointed to his strong electoral mandate, in which his bloc won nearly three times more seats than its closest challenger.

His remarks came after a farcical parliamentary session in which Iraq's various factions -- many of which strongly oppose him staying -- failed to unite and choose a speaker.

That sparked criticism from abroad and from the country's top Shiite religious leader.

Further highlighting Iraqi disunity, the leader of the country's autonomous Kurds has called on their lawmakers to work towards holding a referendum on independence.

The Kurds' long-held dream of statehood has been advanced by the latest crisis, with their forces having moved in to take control of disputed territory they want to incorporate over Baghdad's strong objections.

IS, the jihadist group claiming world leadership
Baghdad (AFP) July 05, 2014 - The Islamic State (IS) jihadist group which spearheaded a sweeping militant assault that overran swathes of Iraq is now claiming leadership of the world's Muslims.

Known for its ruthless tactics and suicide bombers, IS has carried out frequent bombings and shootings in Iraq, and is also arguably the most capable force fighting President Bashar al-Assad inside Syria.

But it truly gained international attention last month, when its fighters and those from other militant groups swept through the northern Iraqi city of Mosul, then overran swathes of five provinces north and west of Baghdad.

The group led by "caliph" Abu Bakr al-Baghdadi, and backed by thousands of fighters in Syria and Iraq, some of them Westerners, appears to be surpassing Al-Qaeda as the world's most dangerous and influential jihadist group.

In a sign of IS confidence, the hitherto secretive Baghdadi made an unprecedented public appearance in the militant-held north Iraq city of Mosul, ordering Muslims to obey him, according to a video distributed online on Saturday.

If authenticated, the recording would seem to be the first known footage of the jihadist leader, who espouses an extreme form of Islam and aims to return conditions in conquered territory to an approximation of those in the early years of the religion.

In a June 29 audio recording, IS spokesman Abu Mohammad al-Adnani declared Baghdadi "the caliph" and "leader for Muslims everywhere", referring to a system of rule last used almost 100 years ago before the collapse of the Ottoman Empire.

Western governments fear IS could eventually strike overseas, but their biggest worry for now is its sweeping gains in Iraq and the likely eventual return home of foreign fighters.

IS appears to have attracted more foreigners than any of the rebel groups fighting in Syria, and unlike other groups fighting Assad, has sought to appeal to non-Arabs by releasing English-language magazines and videos in English, or with English subtitles.

Much of the IS appeal stems from Baghdadi himself -- he is touted as a battlefield commander and tactician, a crucial distinction compared with Al-Qaeda chief Ayman al-Zawahiri.

"If you were a guy who wanted action, you would go with Baghdadi," said Richard Barrett, a former counter-terrorism chief at MI6, Britain's foreign intelligence service.

At the time Baghdadi took over what was then known as the Islamic State of Iraq, or ISI, in May 2010, his group appeared to be on the ropes, but the group has since bounced back, expanding into Syria in 2013.