The Islamic State of Iraq and the Levant 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, ISIL 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 this month, when its fighters and those from other militant groups swept through the northern city of Mosul, then overran major areas of five provinces north and west of Baghdad.
ISIL is led by the shadowy Abu Bakr al-Baghdadi and backed by thousands of Islamist fighters in Syria and Iraq, some of them Westerners, and it appears to be surpassing Al-Qaeda as the world's most dangerous jihadist group.
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In a sign of the group's confidence, it has now expanded its claim of leadership to encompass all the world's Muslims.
In an audio recording distributed online on Friday, ISIL's spokesman Abu Mohammad al-Adnani declared Baghdadi "the caliph" and "leader for Muslims everywhere".
"The Shura (council) of the Islamic State met and discussed this issue (of the caliphate)... The Islamic State decided to establish an Islamic caliphate and to designate a caliph for the state of the Muslims," Adnani said.
He was referring to a system of rule last used to govern a state almost 100 years ago, before the collapse of the Ottoman Empire.
An ISIL fighter, third from right, distributes copies of the Quran in Mosul, Iraq. Photo / AP
Western governments fear ISIL could eventually emulate Al-Qaeda and strike overseas, but their biggest worry for now is its sweeping gains in Iraq and the likely eventual return home of foreign fighters attracted by ISIL and Baghdadi.
Among them are men like Mehdi Nemmouche, a 29-year-old Frenchman who allegedly carried out a deadly shooting on a Jewish museum in Belgium after spending a year fighting with ISIL in Syria.
12,000 foreign fighters
The Soufan Group, a New York-based consultancy, estimates that 12,000 foreign fighters have travelled to Syria, including 3000 from the West. ISIL appears to have the greatest appeal, with King's College London professor Peter Neumann estimating around 80 percent of Western fighters in Syria have joined the group.
Unlike other groups fighting Assad, ISIL is seen working towards an ideal Islamic emirate. Compared with Al-Qaeda's franchise in Syria, Al-Nusra Front, it has lower entry barriers.
ISIL has also sought to appeal to non-Arabs, publishing English-language magazines, after having already released videos in English, or with English subtitles.
ISIL militants take aim at captured Iraqi soldiers. Photo / AP
The jihadist group claims to have had fighters from Britain, France, Germany and other European countries, as well as the United States, and from the Arab world and the Caucasus.
Much of the appeal also stems from Baghdadi himself. The ISIL leader is touted as a battlefield commander and tactician, a crucial distinction compared with Al-Qaeda chief Ayman al-Zawahiri.
"Baghdadi has done an amazing amount - he has captured cities, he has mobilised huge amounts of people, he is killing ruthlessly throughout Iraq and Syria," said Richard Barrett, a former counter-terrorism chief at MI6, Britain's foreign intelligence service.
"If you were a guy who wanted action, you would go with Baghdadi," Barrett told AFP.
ISIL leader Abu Bakr al-Baghdadi. Photo / AFP
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, after the "surge" of US forces combined with the shifting allegiances of Sunni tribesmen to deal him a blow. But the group has bounced back, expanding into Syria in 2013.
Baghdadi sought to merge with Al-Nusra, which rejected the deal, and the two groups have operated separately since.