Foreign jihadist fighters are using safe houses in southern Turkey to cross into Syria to fight against government forces, the BBC has learnt.
A man running one such house near the border town of Reyhanli said more than 150 people - including up to 20 Britons - had used it in the past three months.
The route through Turkey used by al-Qaeda-linked foreign jihadists is now becoming increasingly organised.
Opposition activists say jihadists are destroying the Syrian revolution.
'Not true Muslims'
The man in charge of the safe house near Reyhanli told the BBC's Richard Galpin that "more than 150 people stayed at the house" in the past 90 days.
"Between 15 and 20 were British. It's all done through invitations from friends".
He added that jihadists usually "stay for a day or two before crossing into Syria and stay on the way back when they are waiting for flights back to their home countries".
One such fighter from France told our correspondent that "there are thousands of us, literally from every corner of the world".
"And we are all al-Qaeda," he added.
The jihadist, a former student in France, said he had joined a brigade which had 8,000 men.
He added that the brigade had recently pledged allegiance to the radical Islamist organisations called the Islamic State of Iraq and Greater Syria
Over the past year, thousands of foreign fighters - including about 300 British nationals - have poured into Syria to fight President Bashar al-Assad's forces.
But Syria's original armed opposition say the jihadists are not just fighting the regime but are also systematically targeting Free Syrian Army fighters.
A former FSA commander said he had to flee to Turkey after his unit had been captured by jihadists.
"They told us we were not true Muslims," the commander said.
"I saw how they beat my friends with iron bars, smashed their faces with ammunition boxes and then killed them. The floor was covered with blood.
"We made the revolution for freedom and equality but the jihadists don't want this. They've come to destroy Syria."
The commander added that he was one of the few men from the unit to survive.