Not only individual fighters, but entire units have joined the small but powerful al Qaeda-linked groups - the Nusra Front and the Islamic State of Iraq and the Levant (ISIL) - in recent days, according to sources inside Syria.
"This is a sign the radical groups are still growing in power. This region could fall to the jihadists," said an activist in the eastern town of Raqqa, who asked not to be identified. "We may see this become a trend."
Clashes have been intensifying between Nusra or ISIL and the less effective but more moderate forces make up the majority of opposition fighters, especially in opposition-held territory along Syria's northern and eastern borders.
On Friday, al Qaeda-linked fighters and a rival Syrian rebel group declared a truce after two days of clashes near the Turkish frontier in which the Islamic militants overran a border town, rebels had said.
At least two entire rebel brigades are said to have joined the Nusra Front in the opposition-held province of Raqqa, which borders Turkey.
One of the groups, the Raqqa Revolutionaries, has about 750 fighters in total, according to a source close to pro-Islamic forces, who spoke on condition of anonymity.
Another group, the God's Victory Brigade, said in a statement on Facebook that all of its leaders and fighters had pledged loyalty to Nusra Front.
"God's Victory Brigade, which is comprise of 15 battalions, had pledged its allegiance to the Nusra Front, giving complete submission (to it) in times of hardship and of ease," it said.
A video uploaded by activists from Raqqa on Friday showed a massive convoy of fighters on cars and trucks with artillery and machine guns as they waved black flags. The video's title said it showed a newly unified force of Nusra fighters and other rebel battalions who had recently pledged loyalty.
Western forces have been wary of giving further support or weapons to opposition forces who are not only plagued by internal divisions, but the rising influence of al Qaeda groups.
Sporadic clashes between Islamic hardliners and more moderate rebels are increasingly frequent and activists fear that is weakening the two-and-a-half-year revolt against President Bashar al-Assad. The uprising began as peaceful protests against four decades of Assad family rule but has degenerated into a war that has killed more than 100,000.
While some tensions stem from contrasting ideological outlooks, most rebel-on-rebel fighting is more about control of territory and the spoils of war.
Some activists said the new loyalty pledges were timed to combat increasing hostility from rival rebel groups, including the Supreme Military Council, the armed wing of the opposition's Western-backed umbrella leadership abroad.
Many Syrian rebels are attracted to radical units because they are generally more effective than the moderate forces that have Western backing but receive only halting military aid.
Extremist groups have steady, private sources of funding and incorporate experienced militants, many of them from abroad, who have fought US forces in Iraq or Aghfanistan.
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It would be quite fair for both of them stop the inter-conflicts to spare lives of innocent young children & women
Extremism almost always is born out of oppression/occupation. Democratic countries with social/economic justice rarely have extremist problem.
This is direct result of Dictatorship by Al Assad Clan for last 42 years. Most Syrians are moderate Muslims( both pro Assad/dictatorship and Pro Democracy forces), but now this has unfortunately become a sectarian war and extremists on both sides are stronger than their secular counterparts. We increasingly see Hezbollah/Iranians on government side and Salafists on opposition side gaining more and more control.
If Bashar who is a medical doctor had given his countrymen their basic right of democracy and chance of electing their own representatives and leader, we could have avoided this mess.
WHO says that the West supports al-qaeda in syria?. they dont and will never do. the west are supporting bashar. Bashar is the wests man, its very clear. Now that al-aqaeda is disturbing the Wests interests in syria is another thing but have an idea that the West is supporting al-qaeda is very strange thinking. it seems that the West is loosing hold in syria if you ask me.
This is why we have been saying that supporting al-aqaeda in Syria by the west will make their earlier actions against terrorism as highly doubtful. This unification has given all the more good reasons to clean Syria of the terrorism.
Its only the 800 plus Salafists from Europe who have joined Al-Qaeeda. All of Europe hopes they never come back ! In fact Pakistan should also shove all Taliban into this war as fodder. Who cares whether it is Al-Qaeeda or Mussad or CIA or MI5 they join.