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.