A new report is criticising the United States for its "misguided" strategy of focusing on the Islamic State (ISIS), saying al-Qaeda's Syrian affiliate, Jabhat al-Nusra, is much more dangerous in the long run than the latter.

The report released last week by the Institute for the Study of War and American Enterprise Institute describes both al-Nusra and ISIS as "existential" threats to the U.S.

It was authored by Fred Kagan, considered an architect of the 2007 "surge" strategy in Iraq, which boosted American troops and engagement with local tribes to stabilise that country, and Kim Kagan, a former adviser to Gen. David Petraeus on Afghanistan strategy and the president of the Institute for the Study of War.

"Any strategy that leaves Jabhat al-Nusra in place will fail to secure the American homeland,'' the report said.

"While ISIS is flashier ... both represent an existential threat, both wish to attack the homeland, both seek the mobilisation of Muslim communities against the West," it added.

The report cautioned that al-Qaeda's Syrian branch represented "a longer-term and more intractable threat than ISIS" and is "quietly intertwining" itself with the Syrian population and Syrian opposition to expand their reach.

In a speech delivered on Wednesday, U.S Army Gen. Mark Milley clarified that only Moscow constituted a potential "existential'' threat due to its possession of a large nuclear arsenal capable of striking the U.S.

But the report argued that ISIS and al-Nusra attacks "could threaten the global economy and provoke Western societies to impose severe controls on ... freedoms and civil liberties," thereby endangering "American values and way of life.''

Meanwhile, Syrian President Bashar al-Assad is set to meet with select rebel groups this month in Geneva, Switzerland for peace talks. ISIS and al-Nusra will not be participating, CNN reported.

Al-Nusra emerged in late 2011 during the early days of the Syrian civil war and was initially largely made up of battle-hardened Syrians who had traveled to Iraq to fight U.S. troops during the American engagement there, CNN said.

Since its inception, the group has already attracted foreign fighters and gained control of northwestern Syria. It has reportedly emerged as one of the most effective groups fighting the Syrian regime.

Nick Heras of the Center for a New American Security said the group holds "coercive power" over several opposition groups, serving as a sort of "kingmaker.''

Despite al-Nusra's potential threat to the homeland, the DNI's 2015 threat assessment report to the Senate Armed Services Committee only listed the group under regional threats and not named in the global threats section.

The State Department's 2014 Country Reports on Terrorism also labels al-Nusra a threat to "the Syrian opposition, Syrian civilians, and other states in the region.''

"Al-Nusra has made a tactical decision not to attack the West for the time being because its leadership's priority is preserving success in Syria and avoiding being targeted by the U.S," the authors said.