They're warning that the decision by the coalition bombing Syria to focus only on ISIS risks assisting al-Qaeda, as well as the regime of President Bashar al-Assad.
Jabhat al-Nusra (The Support Front) is made up of fighters who remained loyal to Al Qaeda's leadership when the Islamic State's leader, Omar al-Baghdadi, announced a split from the older jihadi group at the beginning of 2014.
Baghdadi declared the refounding of the Muslim caliphate and named himself Caliph Ibrahim, calling on all Muslims in the world to pledge allegiance. Al-Nusra's fighters do not recognize that caliphate and have clashed with ISIS on several occasions.
ISIS made dramatic early gains, seizing much of eastern Syria as well as northern and western Iraq. But it has recently found itself on the defensive as it has been targeted by western, Iraqi and Iranian forces.
Meanwhile, Al Nusra has been growing, said Fawaz Gerges of the London School of Economics, one of the world’s foremost Middle Eastern scholars.
“Ninety-nine per cent of the airstrikes have targeted ISIS. Al-Nusra Front has been gaining momentum and influence in many parts of Syria, because the fight basically targets its rival."
Canada: No plan to bomb al-Qaeda
Canada's defence minister Jason Kenney says Canada is not planning to strike al-Nusra.
"Our fight is against [ISIS]. This is the organization that has declared war on Canada, encouraged its followers to attack Canadians wherever they are, and that has invaded Iraq, based out of eastern Syria. The other organizations like the al-Nusra Front are operating primarily in central and western Syria far from the Iraqi border."
Canada's legal mandate depends partly on the fact that Iraq has requested our assistance to defend itself against ISIS. Al-Nusra has not attacked Iraq.
However al-Nusra is part of al-Qaeda, which has also encouraged its followers to attack Canadians. Indeed al-Qaeda has killed far more westerners than ISIS, and continues to be behind some of the most lethal attacks, such as the Charlie Hebdo massacre in Paris.
Embedded within al-Nusra is a group of older al-Qaeda fighters the Pentagon has dubbed the "Khorasan cell," whose members have years of experience fighting in Afghanistan, Pakistan and Iraq.
This group was targeted, unsuccessfully in the first wave of U.S. airstrikes over Syria, but since then the focus has been on ISIS.
Al-Nusra has subtle growth strategy
Where ISIS is led by Iraqis and has recruited large numbers of foreigners, Al-Nusra is made up mostly of Syrians. It has avoided the provocative actions of ISIS, such as gory videos of executions. Though it has active propaganda channels, they focus on showing al-Nusra's battles against the Assad regime.
Observers say this strategy has made al-Nusra far more popular within Syria than ISIS.
"Jubhat al-Nusra is going to increasingly occupy the dominant position in rebel ranks," says Syria analyst Jennifer Cafarella of the Institute for the Study of War in Washington, D.C. "Even in the absence of action against ISIS, we really are witnessing a moment in which al-Qaeda is ascendant in Syria."
However, al-Nusra shares ISIS's religious fanaticism. It has ordered the mass conversion to Sunni Islam of the minority Druze community in villages near the Syria-Lebanon border, forcing them to destroy their ancient shrines.
It has also turned its guns against moderate and secular rebel groups. Last month it overran the western-backed Hazm Movement around Aleppo. Hazm members complained that U.S. aircraft did nothing to protect them from al-Nusra, which seized a huge haul of U.S.-supplied weaponry including TOW missiles.
And with ISIS in retreat in Iraq, al-Nusra has surrounded the strategic Syrian city of Idlib, currently held by the Assad regime. The city is widely expected to fall.
Last man standing?
Cafarella says the West has allowed itself to be distracted by ISIS's declaration of a caliphate, while overlooking the danger of its older nemesis, al-Qaeda.
"Nusra wants to create an Islamic emirate that is responsive to al-Qaeda," says Cafarella. "This makes Nusra inherently a threat to the West."
Fawaz Gerges said that if current trends continue, al-Nusra may be the ultimate winner of the Syrian civil war, creating a vast new territory for al-Qaeda.
"The irony is that even though the international coalition might in two or three years defeat ISIS in Iraq, and begin the fight against ISIS in Syria, al-Nusra Front could outlast ISIS," he said.
"The international community really has lost sight of the fact that the al-Nusra Front is as dangerous, is as extremist, even though it presents a more moderate and rational face than its Islamist rival ISIS."