By Nic Robertson and Paul Cruickshank

A jihadist group with links to al Qaeda has become the most effective of the different factions fighting the 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.

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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.

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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.

Last month al-Nusra launched two of its most ambitious operations to date. On December 10, the group occupied parts of a military base near Aleppo and two days later claimed responsibility for a coordinated suicide and car bomb attack on the heavily guarded Interior Ministry in the capital.

Al-Nusra's signature tactic, like that of AQI, is using large car and truck bombs driven by suicide bombers. The group has launched several such attacks against security installations in Damascus and Aleppo, sometimes as part of a coordinated assault involving gunmen.

Benotman says that last Summer al-Nusra launched a recruitment drive for suicide bombers and began stockpiling trucks and explosives. He says that weapons shortages among rebel groups means that al-Nusra's campaign of suicide bombings has allowed it to punch above its weight.

Last week al-Nusra demonstrated the lethality of a new tactic - driverless car bombs operated by remote control, Benotman told CNN. He says the technology was used to destroy a gate at an airbase in Idlib and will raise fears that it could one day be used in an attack in the West.

If al-Nusra's fighting strength is some 5,000 members, as the Quilliam report estimates, that would be comparable to U.S. government estimates of AQI at the peak of the Iraq insurgency. But rebel commanders say that the group makes up less than 10% of the brigades fighting the regime.

While al-Nusra is mainly made up of Syrians, it includes a significant number of fighters from other Arab countries. In recent months a growing number have arrived from Saudi Arabia and the Gulf, but Iraqis and Jordanians constitute the majority of foreign fighters.

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Cooperation with other rebels

In recent months, videos featuring rebels fighting in Syria have increasingly featured joint-operations between al-Nusra and other rebel groups.

According to the Quilliam Foundation report, al-Nusra often cooperates with other jihadist and Islamist groups such as Sukour al-Sham, which has several thousand fighters, and even with the Free Syrian Army, in a number of strategic battles, though joint operations between these two groups have not been widespread.

According to Benotman, a significant number of Jihadists fighting with other rebel outfits are wary of al Qaeda's hard-line ideology, but al-Nusra has sought to allay concerns by keeping its brand separate from al Qaeda, avoiding targeting civilians, and refraining from spelling out its true agenda.

"Preserving good relations with the other groups and treating them well and turning a blind eye to their mistakes is the foundation in dealing with the other groups, as long as they don't change," al-Nusra leader Mohammed al-Jawlani said in a December audio tape, according to a translation by the SITE Intelligence Group.

Al-Nusra and nine other local Jihadist brigades announced last month they were forming a regional unified command structure called the Mujahideen Shura Council in Deir el-Zour.

Yet according to Benotman's report, al-Nusra has not yet formed any such coalitions with larger Islamist rebel outfits such as Ansar al-Islam, Ahrar al-Sham, and the Deir Ezzor Revolutionary Council, three groups which previously joined together to form the "Liberation Front."

A counterproductive designation?

According to the Quilliam study, "the designation (by the U.S.) of al-Nusra as a terrorist organization has only served to reinforce jihadist support for the group.

Nada Bakos, the former CIA agent agreed, telling CNN the designation may elevate al-Nusra's status amongst Jihadists worldwide, increasing funding and recruitment for the group.

Benotman's study describes relations between al-Nusra and the FSA as mixed, with both realizing they need each other in the short term to topple al-Assad.

"Some FSA brigades threaten to work with al-Nusra if the West does not provide enough weapons while others see al-Nusra as trying to exploit the revolution for their own ends, instead of working for the good of the country. Jabhat al-Nusra and the Free Syrian Army are wary of one another, as they are already vying for popularity amongst the population," Quilliam says.

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Strategy

Bakos, the former CIA official says AQI and al-Nusra are likely replicating the flexible, decentralized, and resilient external operations networks established by al-Zarqawi in the region, and that makes them a force to be reckoned with. Benotman says the al-Zarqawi networks never really went away.

Analysts believe al-Nusra's hostility to the West could create an "over-the-horizon" threat to the United States and its allies if the group is able to secure a foothold in Syria and across the Levant.

In such a scenario al Qaeda aligned groups would be operating within touching distance of borders of Israel, improving their potential to launch a direct attack against the country, long a key proclaimed objective of the terrorist network's leader Ayman al-Zawahiri.

The Quilliam Foundation report is sobering reading at a time when increasing sectarian tension and regime brutality in Syria are playing into al-Nusra's hands.

Benotman believes al-Nusra doesn't want a quick end to the al-Assad regime.

"The longer the conflict goes on, the stronger they will get," he told CNN.