Most of them avoid reporters like the plague but in "liberated" northwestern Syria, it is difficult not to run into foreign jihadist fighters, both on the front lines and at rebel bases.
"Secrecy shrouding the activities of foreign militants makes it extremely difficult to assess with any accuracy their extent, location and potential ramifications," the International Crisis Group said in a report.
But while President Bashar al-Assad's domestic foes have tried for months to downplay the impact of outsiders, now "foreign militants have had more direct involvement, fighting alongside Syrian insurgents," the Brussels-based group added.
The small town of Atme on the border with Turkey serves as a hub for foreign volunteers in the 21-month insurgency that has cost tens of thousands of lives.
They cross from the Turkish city of Antakya, travelling there from far and wide to join the "holy war".
AFP correspondents have spotted them walking in the alleyways of Antakya's old bazaar or in cheap hotels near the bus station.
In September, three well-built men were seen in the city wearing North African djellabas (robes) over trousers cut above the ankles in line with the tradition of early Muslims, the mark of hardline Salafists.
The trio stood out with their beards and attire as foreign volunteers rather than locals, while the brand-new trekking boots and backpacks suggested they were probably European converts to Islam.
These days candidates for jihad are more discreet.
They make their way to the Turkish border town of Reyhanli before smugglers guide them across the border to Atme, a key node in the embattled country's revolt and rallying point for foreign fighters.
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An AFP correspondent made the journey into Syria through olive groves and holes in the barbed wire alongside one such fighter. A quiet Egyptian, he said he had made the journey to "help my Muslim brothers."
Another fighter, 26-year-old Anas from Algeria, was already a war veteran.
He fought in his homeland's Kabylie region, east of Algiers, as well as in Kashmir. Now, he was on his way to join a rebel Free Syrian Army unit manned by the jihadist Al-Nusra Front near the town of Harem.
Despite the dangers ahead, Anas was happy that he could at least speak French with the AFP correspondent.
For Abdel Taha, who said he hails from the Islamic Movement of Uzbekistan, language has been a problem, as he takes part in the rebel siege of the Sheikh Suleiman army base near the northern city of Aleppo.
A speaker of neither Arabic nor English, he resorts to phrases which cause mirth among his comrades, including "kill the Christians and the unbelievers." To the AFP correspondent, he asked in a semi-serious voice: "Are you a Muslim?"
In the heavily bombed town of Maaret al-Numan, a Libyan with African features welcomed the AFP journalist at the front line. "Do you speak Italian?" he asked in the European language he knows best, before rushing back into the fray.
In the Jebel Akrad mountains, four Saudi men run the online websites of Islamist groups such as the Al-Nusra Front. They live in an abandoned apartment in the town of Salma, with a rocket-propelled grenade sitting in the living room near a copy of the Koran.
Asked what they are doing in Syria, they replied: "Tourism."
Back in Atme, black-clad men speaking English, some of them Europeans, others east Africans, can be seen from a kebab shop on the main road. But they are not approachable as they emerge from a nearby Al-Nusra Front house.
At the end of November, more than 100 of them gathered in front of a mosque, under a sea of black Islamic flags after a Palestinian imam had preached to local residents of the benefits of their presence in the town.