The London-based global defense consultancy said about 10,000 of the estimate 100,000 insurgent fighters are linked to al-Qaida, mainly Iraqis but with large numbers of foreigners, and 30,000 to 35,000 belonging to "powerful factions" that are fighting for an Islamic state within a larger Middle East caliphate stretching from the Atlantic to the Indian Ocean.
In addition, the study, reported on by the British newspaper The Daily Telegraph Monday and slated for release later this week, says here are "at least a further 30,000 moderates belonging to groups that have an Islamic character."
That would leave only 25,000-30,000 fighters with more secular, nationalist groups, or a scant 25-30 percent of the rebel force that could be considered friendly to the West.
IHS Jane's said the study was based on "intelligence estimates and interviews with activists and militants."
The Telegraph added the report "accords with the view of Western diplomats who estimate that less than one-third of the opposition forces are 'palatable' to Britain while American envoys put the figure even lower."
If the numbers are anywhere near accurate, and there seems little reason to question the report's conclusions, it's bad news for the U.S. administration of President Barack Obama, which wants to see a Western-oriented government in Damascus to block Iran's drive to extend its dominion to the eastern Mediterranean and Israel's backyard.
But then the prospect of having Sunni Muslim zealots controlling Syria, Shiite Iran's strategic gateway to the Levant, isn't good news for Tehran either.
That could lend weight to reports Iran's Revolutionary Guards, particularly its elite Al-Quds Force, is currently engaged in beefing up its military support for Assad's beleaguered regime with significant numbers of Shiite militiamen from Iraq, Syria's eastern neighbor and Iran's vital land corridor to Syria.
It could also help explain how Assad has succeeded in regaining the upper hand in his 2 1/2-year-old war with his opponents in the last six months, and in backing down on surrendering his chemical weapons, although key ally Russia would seem to have been the primary influence on that score.
The Jane's report indirectly underlines the consequences of the U.S. reluctance to support the so-called moderate rebel groups with large-scale arms shipments, including advanced systems like anti-aircraft missiles which the desperately need to counter Assad's air force, one of his key military assets.
This because the West, particularly the United States, fears these weapons would end up with the Sunni jihadists who'd use them at some point against their donors.
Charles Lister, head of Jane's terrorism center and the report's author, observed: "The insurgency is now dominated by groups which have at least an Islamist viewpoint on the conflict.
"The idea that it is mostly secular groups leading the opposition is not just borne out."
This conclusion cannot be news to U.S. intelligence services or their European and Arab counterparts, and is not going to change just because Assad is prepared, presumably with Russian and Iranian pledges of support, to surrender his considerable chemical weapons arsenal to the United Nations in return for not being hit by U.S. military strikes for gassing his own people in August.
Even if the Americans don't step up arms supplies and training for the non-Islamist rebels, it's likely that Saudi Arabia, leader of the Sunni world and locked in a widening conflict with Shiite Iran, will.
Riyadh's track record of covert operations supporting the war against the Soviets in Afghanistan (1979-89) and the recent escalation in its arms shipments to the so-called moderate rebels in Syria -- who now seem to spend as much time fighting the Islamists as they do the regime -- would indicate that this is so.
Jane's says that the main jihadist groups, the al-Nusra Front and the Islamic State of Iraq and the Levant, now dominate the battlefield and control income-generating resources in northern Syria, including oil, natural gas and grain.
"If the West looks as though it is not interested in removing Assad, moderate Islamists are also likely to be pushed further toward extremists," Lister warned.