The Islamic State of Iraq and the Levant, which dealt a spectacular blow to Baghdad's Shiite-led government by grabbing Iraq's second city Mosul, now poses a threat across the Middle East.

Known for its ruthless tactics and suicide bombers, ISIL has already controlled the Iraqi city of Fallujah for five months, and is also arguably the most capable force fighting President Bashar al-Assad inside Syria.

Its takeover of Mosul on Tuesday prompted the United States to voice deep concern about the "extremely serious" situation and warn the jihadist Sunni group poses "a threat to the entire region".

UN chief Ban Ki-moon's spokesman said he was "gravely concerned by the serious deteriorating of the security situation in Mosul".

ISIL is led by the shadowy Abu Bakr al-Baghdadi and backed by thousands of Islamist fighters in Syria and Iraq, many of them Westerners, and it appears to be surpassing Al-Qaeda as the world's most dangerous jihadist group.

Western governments fear it could eventually emulate Al-Qaeda and strike overseas, but their biggest worry for now is likely the 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 last month after spending a year fighting with ISIL in Syria.

- Working towards Islamic emirate -

The Soufan Group, a New York-based consultancy, estimates that 12,000 foreign fighters have travelled to Syria, including 3,000 from the West.

And 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 that straddles Syria and Iraq. And 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, recently publishing two English-language magazines, having already released videos in English, or with English subtitles.

The jihadist group claims to have had fighters from the 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.

"For the last 10 years or more, (Zawahiri) has been holed up in the Afghanistan-Pakistan border area and hasn't really done very much more than issue a few statements and videos," said Richard Barrett, a former counter-terrorism chief at MI6, Britain's foreign intelligence service.

"Whereas 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.

"If you were a guy who wanted action, you would go with Baghdadi," Barrett told AFP.

Baghdadi apparently joined the insurgency that erupted in Iraq soon after the 2003 US-led invasion.

In October 2005, American forces said they believed they had killed "Abu Dua," one of Baghdadi's known aliases, in a strike on the Iraq-Syria border.

But that appears to have been incorrect, as he took the reins of what was then known as the Islamic State of Iraq, or ISI, in May 2010 after two of its chiefs were killed in a US-Iraqi raid.

Since then, details about him have slowly trickled out.

- Bearded man, suit, tie -

In October 2011, the US Treasury designated him as a "terrorist" in a notice that said he was born in the Iraqi city of Samarra in 1971.

And earlier this year, Iraq released a picture they said was of Baghdadi, the first from an official source, depicting a balding, bearded man in a suit and tie.

At the time Baghdadi took over, 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.

Zawahiri has urged ISIL to focus on Iraq and leave Syria to Al-Nusra, but Baghdadi and his fighters have openly defied the Al-Qaeda chief and, indeed, have fought not only Assad, but also Al-Nusra and other rebel groups.