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.

Formerly known as ISIS and 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 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, as well as 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.