A jihadist group with links to al Qaeda has become the most effective of the different factions fighting the Syrian regime, according to a new analysis, and now has some 5,000 fighters.

The group is Jabhat al-Nusra, which was designated an al Qaeda affiliate by the United States government last month. It is led by veterans of the Iraqi insurgency "and has shown itself to be the principal force against Assad and the Shabiha," according to the study.

CNN obtained an advance copy of the analysis, set to be released Tuesday by the Quilliam Foundation, a counterterrorism policy institute based in London.

"The civil war in Syria is a gift from the sky for al-Nusra; they are coasting off its energy," the lead author of the report, Noman Benotman, told CNN.

Benotman, a former prominent Libyan Jihadist who was personally acquainted with al Qaeda's top leaders including Osama bin Laden and Ayman al-Zawahiri, consulted Western and regional intelligence officials as well as jihadists in Syria , including "al-Nusra sources."

And at a time of optimism that the global threat from al Qaeda terrorism has crested, the study will fuel anxiety in Western capitals that a powerful al Qaeda affiliate may become entrenched in the heart of the Arab world, creating deep challenges in any post- al-Assad Syria, and a new threat to international security.

Founding meetings

Al-Nusra, according to the report, is a Syrian offshoot of al Qaeda in Iraq, aka AQI, the terror outfit founded by the Jordanian Abu Musab al-Zarqawi.

AQI was rebranded the "Islamic State of Iraq" after al-Zarqawi was killed by a U.S. missile strike in 2006. Since the pull-out of U.S. troops from Iraq, ISI has regained strength, feeding off the continued political and sectarian turbulence in Iraq.

When designating al-Nusra a terrorist group in December, the U.S State Department cast the group as "an attempt by AQI to hijack the struggles of the Syrian people for its own malign purposes."

"AQI emir Abu Du'a is in control of both AQI and al-Nusra. Du'a also issues strategic guidance to al-Nusra's emir, Abu Muhammad al-Jawlani, and tasked him to begin operations in Syria," the State Department said.

Benotman says that while Abu Du'a still has significant influence over al-Nusra, the key player in the group is al-Jawlani, a veteran Syrian jihadist who he says appears to have almost certainly been a former close associate of al-Zarqawi.

Al-Jawlani's "leadership is uncontested because of his experience in Iraq," the Quilliam Foundation report found. According to Benotman, al-Jawlani has taken painstaking measures not to reveal his real identity -- including wearing a mask to meetings with some of al-Nusra's senior operatives. He was also masked when al-Nusra released a video in January 2012 to announce its formation.

AQI had built up an infrastructure in Syria, establishing safe-houses in Syria from which thousands of volunteers -- including many Syrians -- traveled to fight in Iraq. Al-Zarqawi's Syrian commanders were also the key channel for financial contributions from the Saudi and Gulf region.

Nada Bakos, a former CIA agent who for several years was the chief targeting officer tracking al-Zarqawi, told CNN that from the early days Syrians were amongst the inner circle of his network. "Some of these commanders are probably now part of al-Nusra," she said.

One Syrian among the inner circle of AQI was Sulayman Khalid Darwish. He's been reported killed in Iraq, but intelligence sources tell CNN his fate remains uncertain, raising the possibility he may now be playing a leadership role in al-Nusra.

According to Benotman, the ultimate aim of al-Nusra is the creation of an Islamic State in Syria and the Levant. To begin with, it set about recruiting fighters and training them, collecting weapons and creating safe havens.

The group suffered a severe setback in April 2012 after the arrest of an operative led to a significant number of members being detained in Damascus, but the group subsequently rebuilt its operations, placing greater emphasis on operational security, Benotman told CNN.

One precaution al-Nusra has taken is communicating through messengers rather than electronically, according to Benotman. "Their operational security is some of the best I've ever seen," he told CNN.

In addition, al-Nusra is "very selective about initiating new members, requiring "tezkiyya," or personal assurance, from two commanders on the front line stating that the recruit has the necessary skills, religious commitment and attitude to join the group," the Quilliam study says.

From clandestine cells to insurgency

According to the U.S State Department, al-Nusra has claimed responsibility for nearly 600 attacks -- "ranging from more than 40 suicide attacks to small arms and improvised explosive device operations -- in major city centers including Damascus, Aleppo, Hama, Dara, Homs, Idlib, and Dayr al-Zawr."

Benotman says the group has also carried out executions of media professionals and assassinations of military officers and members of the pro-al-Assad Shabiha militia.

Al-Nusra also focuses on taking control of towns near major highways to control movement; it controls the highway between Aleppo and Hasakah, an important route to Iraq, according to the Quilliam report.

So far the group has only claimed one attack on Syrian government planes and helicopters which "would seem to demonstrate a lack of man-portable air-defence systems (MANPADs), consistent with the international effort to keep these weapons out of jihadist hands," according to the report.