Homegrown rebels have done most of the fighting against the Syrian government troops. But Islamist militants from abroad, including some with links to al-Qaida, are now joining the fight against the government in growing numbers.
The local rebels are not pleased with this development, and there is growing tension between the groups that share a desire to oust President Bashar Assad but little else.
Until a few weeks ago, the border crossing at Bab al-Hawa on Syria's northern frontier with Turkey was the site of a training camp for a militant Islamist group.
But a rebel with knowledge of the operation to eliminate the al-Qaida-linked group says they are now gone — and he says it's for the good of Syria.
Now, the black banner favored by Islamist militant groups has been replaced at the checkpoint with the flag of Syrian rebels. Syrians crossing the border into Turkey confirmed that just a few of the foreign fighters remain.
"They are still there. We talked to them a little bit," says Abu Mahmoud, a Syrian activist, adding that the leader of the Islamist radicals was assassinated.
First Direct Clash
That leader, Firas al-Abseh — also known as Abu Muhammad — was Syrian. But activist Abu Mahmoud says Abseh's fighters were radicals from abroad; there are reports of fighters from Libya, Tunisia, Saudi Arabia and Jordan.
A rebel source says Abseh trained in an al-Qaida camp in Afghanistan in the 1980s, and that once he was killed, his unit was dispersed and paralyzed.
This was believed to be the first direct confrontation between Syrian rebels and Islamist radicals.
The Farouk brigade, the largest of the homegrown Syrian rebel groups, provoked the clash. The dispute started over a flag, says Adib Shishakli, a Syrian opposition politician.
"They had a disagreement about what flag should be raised over Syria," he says — the flag of the Syrian rebels or the Islamists' black banner.
The flag raised larger concerns, he says, that radicals are trying to hijack the revolution.
"This is not a political playground for everybody to come with his own agenda and try to execute it," Shishakli says.
Fierce Competition For Guns And Money
The killing of an Islamist extremist by Syrian rebels is a sign of the internal dynamic of the fight to topple the regime — and the intense competition for guns and money.
A new study by the London-based Quilliam Foundation, which tracks radical groups, reports that extremists make up less than 10 percent of Syria's rebels, but warns that their influence is increasing because they are better funded and armed.
In a crowded apartment on the Turkish border, Syrian officers who defected recently say more extremists are joining the revolution.
They complain, bitterly, that money is pouring in from private donors who support radical ideologies. Abu Mohammed, a military doctor, blames the West for failing to back Syria's moderate rebel battalions.
"Because when I am desperate and I need weapons, guns, and I find those guys filled with those things, I will join them," he says.
Radicals Give 'Wrong Picture' Of Rebels
When asked whether this gives the revolution a bad name, he responds: "The Western world created al-Qaida in Syria by doing nothing."
Western governments are debating how much support to give the rebels. Concern over extremists is one factor that has made them hesitant.
Syrian rebels at the border say that Abseh, the extremist leader, had to go. Mahmoud Najar, a rebel fighter, says his death sent an important message.
"Because he give a wrong picture about us. The real Islam is not like they understand it," he says of the radicals.
We are not extremists here, says Najar, and these radicals are against us, too.
"They do such wrong things to us. So, they must be captured, all of them," he says.
The rebels at Bab al-Hawa eliminated one extremist group. But as the Syrian conflict grinds on, more are joining the fight.