Yet as recently as early August, the strength and influence of the radical forces appeared to be declining amid Western support for the Free Syrian Army and the mainstream Syrian National Coalition.
The sharp reversal of fortunes, accompanied by a general turn against the exile-led political opposition to Assad's minority Alawite regime, "largely resulted from the Obama administration's decision to cancel planned airstrikes against the Assad regime" over an Aug. 21 chemical attack that reportedly killed 1,400 civilians, Oxford Analytica observed.
That move "shattered the legitimacy of the FSA and the SNC's pro-Western stance, and emboldened hard-liners."
It also deepened a rift between the United States and its longtime allies in Saudi Arabia and the Persian Gulf monarchies that will only benefit Assad.
The U.S. decision to pursue diplomatic efforts with Russia to dismantle Syria's chemical weapons rather than direct military action, and the unexpected opening with Iran, Assad's main ally, under its new reformist president, have aggravated a growing rift between the U.S. and Saudi Arabia.
The kingdom, which has become obsessed with toppling Assad to block Shiite Iranian expansion throughout the gulf, across Iraq and into the Sunni-dominated Levant, is now a major supporter of Islamist groups in Syria.
These developments are likely to doom the already limited expectations of a political breakthrough in the 2 1/2-year-old Syrian war at the upcoming Geneva II negotiations, analysts say.
On top of these developments, analysts say the indications are that foreign Sunni jihadists are flocking in greater numbers to Syria to battle Assad, whose Alawite sect is an offshoot of Shia Islam, than the flow of Islamist fighters into Afghanistan in the 1979-89 war against the invading Soviets.
This, in turn, accelerates the radicalization of the Syrian opposition and at the same time heightens the sectarian nature of the conflict, reflecting the swelling Sunni-Shiite confrontation across the region led by bitter rivals Saudi Arabia and Iran.
That's bad news for the Middle East, particularly Syria's neighbors, Iraq, Lebanon and Jordan, which are being inexorably drawn into the conflict.
Analysts estimate that 5,000 to 10,000 foreign fighters have poured into Syria since the civil war erupted March 14, 2011, to join the jihadist organizations.
The main beneficiaries have been the Islamic State of Iraq and the Levant, an affiliate of al-Qaida also fighting in Iraq, and the al-Nusra Front.
An estimated 40 percent of ISIL's 4,000 to 5,000 fighters are foreign while about 20 percent of al-Nusra's 6,000 to 7,000 men are foreigners.
Both groups are recruiting heavily, using slick propaganda videos online to attract young Muslims, adding a dangerous new dimension to a multisided war and threatening possible terrorist activity when these volunteers return to their home countries.
These groups place great emphasis on suicide operations, which they now routinely film. These tell the stories of the suicide bombers and demonstrate their religious fervor.
The leading French newspaper Le Monde last week quoted France's intelligence service as reporting a significant spike in the numbers of jihadists flocking to Syria, many of them from Europe.
"Nothing like it has ever been seen before, even for Afghanistan," a senior officer said.
John R. Schindler, professor of national security at the U.S. Naval War College and a counterterrorism specialist, said Le Monde's report "paints a dire portrait of the rising number of Westerners going to wage jihad in Syria."
But more than these concerns, most observers are increasingly worried about the widening split between Washington and Riyadh that greatly complicates an already labyrinthine conflict.
Most critically, U.S. President Barack Obama's sudden move away from military action, on which the mainstream rebel had been counting to cover a new offensive, "prompted Riyadh to begin shifting its support away from the pro-Western Syrian opposition and toward Islamist factions," Oxford Analytica noted.
But this is not expected to prevent the jihadists expanding their position "at the expense of mainstream insurgents...
"Radicalization of the insurgency will benefit the Assad regime," as the growth of al-Qaida affiliates "will further undermine international efforts to build a credible opposition that can engage in talks on a political transition."