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Extreme, violent and loyal to al Qaeda, the gunmen from the Islamic State of Iraq and the Levant (ISIL) who last week seized control of the Iraqi cities of Ramadi and Fallujah are refusing to withdraw despite government onslaughts and threats from local tribal leaders.

Now firmly entrenched in the cities that once witnessed some of the bloodiest battles for U.S. forces during the war in Iraq, their offensives have been well planned, coordinated and designed to spread terror.

In Syria, meanwhile, ISIL has become the strongest fighting group in the north of the country. In the last week alone, activists estimate that fighting between it and other rebels fighting the government of President Bashar Assad has killed nearly 500 people. It also claimed responsibility for deadly bombings in Lebanon.

Allegedly bolstered by an influx of foreign fighters, the group that now claims 5,000 members “expanded their influence significantly in 2013,” according to a recent report by military journal "Jane’s Defense Weekly."

Across the region, the ISIL positions itself as a Sunni Muslim vanguard against what it sees as domination by Shiite Muslims and Western governments.

So who exactly is behind this deadly organization and what are their goals?

Who are the Islamic State of Iraq and the Levant (ISIL)?The Islamic State of Iraq and the Levant (ISIL) is an Islamist insurgent group active in both Iraq and Syria that pledged allegiance to al Qaeda in 2004.

Originally known as “al Qaeda in Iraq” the extremist group that believes in the strict enforcement of Sharia law is composed and supported by a variety of insurgent groups and clans loyal to the Sunni branch of the Islamic faith.

The group is thought to be responsible for the deaths of thousands of Iraqi civilians as well as members of the Iraqi government and its international allies.

What are their aims?Their stated aim is to create a Sunni Islamic religious state based on Sharia law, not just in Iraq, but one that straddles the border into Syria's rebel held eastern desert provinces.

Summing up the groups' aims in a June 2013 audio statement, the group’s current leader Abu Bakr Al Baghdadi vowed to erase the “Western imposed border with Syria” and called on his followers to “tear apart” the governments in both countries and their regional backers.

Ultimately, ISIL seeks to create an Islamic emirate from where they would launch a global holy war, according to Raffaello Pantucci, a senior research fellow at British think tank, the Royal United Services Institute.

“Their final goal is to create an Islamic emirate which becomes a piece of territory which they control. And from there they will start to export jihad everywhere else,” Pantucci said. “Their vision is a stepping stone onto a bigger picture. Syria is basically the first stage in the process.”

Where do they come from? The group was established by Jordanian Abu Musab al-Zarqawi, according to Brian Fishman, a counterterrorism fellow at the New America Foundation think tank.

After he was released from prison in his homeland, Zarqawi commanded fighters in Afghanistan, where he met Osama bin Laden, the founder of al Qaeda and mastermind of the Sept. 11, 2001, attacks on the United States.

He reportedly moved to Iraq in 2001, and in anticipation of the U.S. invasion in 2003, built a network of contacts, recruited fighters, and became the default "emir" of Islamist terrorists in Iraq, according to a report from the Center for Strategic and International Studies. His group then targeted international forces, government infrastructure and personnel, aid workers and reconstruction efforts.

Claiming to represent Iraq’s Sunni Muslim’s they also took aim at Shiite Muslims.

In October 2004, Zarqawi declared his allegiance to bin Laden and changed his group’s name to al Qaeda in Iraq (AQI). He soon became Iraq’s most-wanted militant.

After Zarqawi's death in 2006 and with the group's extremist methods losing support among many members of Iraq's Sunni community, AQI “rebranded” to become the Islamic State of Iraq. The Islamic State of Iraq kept up the campaign of coordinated bombings against the Shiite targets across Iraq, before expanding into Syria after the civil war started there in April 2011.

The cross-border move prompted yet another name change and in April 2013 the Islamic State of Iraq and the Levant was born. (Although the group is also sometime referred to as the Islamic State of Iraq and Al-Sham).

The group’s current leader, Iraqi Abu Bakr al Baghdadi, recently claimed to be a descendant of Prophet Muhammad.

Do they pose a threat to the United States?Although some Americans militants are thought to have fought with or alongside the ISIL in Syria, the group poses little threat to the U.S mainland, according to Pantucci.

While members may wish to promote global jihad -- or global holy war -- the group's focus remains on Iraq and Syria and so an attack on American soil seems a distant prospect, he said.

But others aren’t so sure.

F.B.I. Director James B. Comey said this week that the prospect of Americans going to Syria, learning terrorist techniques and returning to the United States is one of his greatest worries.

"We are devoting more resources both to that subject and to that area," he said Thursday during a question-and-answer session with reporters

"People, including Americans, can go to Syria and learn about dangerous techniques, and it's easy to get in and get out. It's a challenge to identify people with bad intent and keep track of them, but we're spending an enormous amount of time on it," said Comey.

What does the future hold?Despite their resurgence in recent years, global intelligence company Stratfor predicted a bleak future for the group.

"For all the dedication and motivation of its fighters, ISIL simply does not have the manpower or the force to overcome its innumerable enemies and achieve its end goals of establishing its version of an Islamic caliphate in Syria and Iraq," it said in an analysis.

Imad Salamey, a professor in international relations at the Lebanese American University, said the Syrian opposition has also realized the ISIL is a major liability -- not just for the image of the opposition but also for turning Syrians against the opposition.

The Associated Press contributed to this report.