Since the beginning of the Syrian conflict in March 2011, between 2,000 and 5,500 foreign fighters, including up to 600 Europeans, have joined rebel forces in the fight against the Assad regime, a new study has found.

The International Center for the Study of Radicalization (ICSR) at King’s College London in the UK published its findings on Tuesday, citing over 450 Western and Arab sources and martyrdom notices on jihadi Internet forums.

Between “140 and 600 Europeans have gone to Syria since early 2011,” said researcher Aaron Y. Zelin, noting that up to 400 remain on the battlefield.

As many as 134 fighters from Britain have fought in Syria, according to the study, with 107 from the Netherlands, 92 from France and 85 from Belgium. The countries with the least fighters in Syria (1 apiece) are Albania, Austria, Bulgaria and Kosovo.

Other countries with fighters include Finland, Ireland, Sweden and Spain.

Most of the Dutch citizens estimated to be fighters have reportedly remained on the battlefield.

“As with previous conflicts, the picture is far from complete and will probably remain so for years to come,” said Zelin. “There is no ‘true census’ of foreign fighters, and publicly available sources are inevitably incomplete.”

He dismissed consistent claims by the Syrian government that the majority of fighters involved in the Syrian conflict were foreigners, citing a “liberal estimate” of 5,500 foreign fighters out of a rebel force some 60,000-strong. Thus the foreigners would represent less than 10%. But “the actual figure is likely to be lower,” said Zelin.

The study found that the motivations for joining the fighting include the “horrific images of the conflict, stories about atrocities committed by government forces, and the perceived lack of support from Western and Arab countries.”

“In many cases, these individuals fully adopt the jihadist doctrine and ideology only when they are on the ground and in contact with hardened fighters,” the report said.

Zelin noted that “not everyone who has joined the Syrian rebels is al-Qaeda, and only a small number may ever become involved in terrorism after returning to Europe.”

However, he added a word of caution for European governments vis-a-vis these fighters: “Individuals with foreign training and/or fighting experience have featured prominently in European based terrorist plots,” and are far more dangerous and sophisticated than domestic cells.

“Based on the sheer scale of recruitment that is currently taking place, European security services are well advised to monitor the situation closely and adopt an intelligence-led, highly discriminate approach toward dealing with returning fighters,” he concluded.

The question of Westerners fighting alongside radical Islamist forces in Syria came to the fore when it was reported that Eric Harroun, a 30-year-old US Army veteran, was fighting with al-Nusra terrorists against the Assad regime. Last week, Harroun was charged in a Virginia court with conspiring to use a rocket-propelled grenade (RPG) while fighting with al-Nusra, which has been linked to al-Qaeda.