Hundreds of al-Qaeda recruits are being kept in safe houses in southern Turkey before being smuggled over the border to wage "jihad" in Syria.
The network of hideouts is enabling a steady flow of foreign fighters to join the country's civil war, according to some of the operatives involved in the process.
These foreign jihadists have now largely eclipsed the "moderate" wing of the rebel Free Syrian Army, which is supported by the West.
Al-Qaeda's ability to use Turkish territory will raise questions about the role the country - a Nato member - is playing in Syria's civil war.
Turkey has backed the rebels from the beginning and its Government has been assumed to share the West's concerns about al-Qaeda. But experts say there are growing fears over whether the Turkish authorities may have lost control of the movement of new al-Qaeda recruits - or may even be turning a blind eye.
Yesterday, Bashar al-Assad, the Syrian President, said any foreign support for rebel "terrorists" must end before any political solution to the country's conflict could succeed.
"Every day there are mujahideen coming here from all different nationalities," said Abu Abdulrahman, a Jordanian managing the flow of foreign fighters. He handles a network of receiving centres in southern Turkey for those wishing to join al-Qaeda's branch in Syria, known as "the Islamic State of Iraq and the Levant".
He spoke from inside an al-Qaeda safe house, using the Skype account of an intermediary and with volunteers from several countries listening in. Once the volunteer reaches Turkey, there are "procedures" before he can join al-Qaeda, explained Abu Abdulrahman: "We have to research you to make sure you are not a spy. If you are foreign, someone in our network needs to recommend you."
These hideouts are generally apartments rented under false names in villages along Turkey's frontier with Syria. The recruits sometimes wait for weeks until they are cleared to cross the border.
The homes are also used as "rest houses" for al-Qaeda fighters from the front line in Syria. Perhaps 10,000 foreign fighters may now be in Syria, according to analysts. Some are hardened veterans of the Iraq war; others are young "first-time jihadists" - with many from Western countries.
Abu Abdullah, an Australian volunteer, said he left to fight in Syria because a "Western lifestyle stands against Islam". He was also repelled by the atrocities of the Assad regime.
"When you see the women and children - any human being - being shot or raped or killed in front of their fathers and families, just because they pray to Allah, you have to be moved by the humanity of it."
Charles Lister, from IHS Jane's, a defence consultancy, said: "There are strong suggestions that the number of foreign jihadists in Syria is increasing. This is likely to do with the ease with which recruits can cross the border."
One analyst said Turkey was "turning a blind eye".
Turkish authorities denied the suggestion.