Baghdad: 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 yesterday 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.
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.