ISIS fighters parade in the northern Syrian c
ISIS fighters parade in the northern Syrian cReuters

World leaders have reacted with alarm to the capture of Iraq's second city by an Al Qaeda offshoot, whose rapidly-expanding control over vasts swathes of Iraq and Syria has seen it morph into a fully-fledged state in all but name.

The takeover of Mosul on Tuesday by the Islamic State of Iraq and the Levant, or ISIS, prompted the United States to voice deep concern about the "extremely serious" situation and warn that the jihadist Sunni group poses "a threat to the entire region".  

UN Secretary General Ban Ki-moon's spokesman also issue a statement, saying the UN chief was "gravely concerned by the serious deteriorating of the security situation in Mosul".

ISIS, which also seized an international airport and captured US-made weapons and equipment during its rout of Iraqi security forces, has dealt a spectacular blow to Baghdad's Shia-led government by cementing its effective control over the entirety of the country's western Nineveh Province, and now poses a clear threat across the Middle East.

In a desperate attempt to fend off the threat posed by the group - who will now surely be eyeing further territorial gains - Iraqi Prime Minister Nouri al-Maliki called a state of emergency and has offered to arm local tribes and citizens opposed to ISIS. 

Known for its ruthless tactics and suicide bombers, ISIS 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. 

ISIS 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 other foreign jihadists, and it appears to be surpassing Al Qaeda as the world's most dangerous terrorist 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 ISIS 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 ISIS in Syria.

Working towards Islamic emirate

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

And ISIS 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, ISIS 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.

ISIS 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 ISIS 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. ISI was Al Qaeda's official branch in that country, but after engaging in the Syrian civil war against Zawahiri's explicit instructions - and then threatening to alienate public support through his extremely brutal tactics - the newly-named ISIS was disowned by the Al Qaeda chief.

Since then, details about Baghdadi have slowly trickled out.  

Bearded man with a suit and 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. More recently, the two have engaged in armed clashes against one another, as rebel groups joined forces to eject ISIS from Syria.

Rebels accuse ISIS of serving the Syrian regime's agenda; the group has embittered its rivals through its ruthless monopolization of territory, attacking rebel forces and killing several prominent rebel leaders in its quest to establish its Islamic state.

Despite losing some territory to Syrian rebels ISIS has consolidated its control over areas still under its control, and Middle East expert Aymenn al-Tamimi observed that previous conservative estimates of the group's strength were clearly vastly underestimating its capabilities.

Speaking to the Washington Post, Tamimi noted that ISIS's ability to brush aside American-trained Iraqi security forces in Mosul at the same time as it is fighting on several other fronts in Syria (against rebels, Kurdish militias and Syrian regime forces), proves its manpower is far greater than the 10,000 fighters it is widely reported to posses.

And speaking to Arutz Sheva yesterday he said its continued success should sound alarm bells.

"If it turns out ISIS have taken exclusive control without sharing with other groups then this is very significant. It means massive expansion of power and resources... so in short, they are just getting stronger and stronger."