(Reuters) - Between 3,300 and 11,000 fighters from more than 70 nations, including a rising number from Western Europe, have joined the struggle in Syria against President Bashar al-Assad, a research group reported on Tuesday.
After security forces repressed peaceful protests against more than 40 years of Assad family rule in 2011, an armed revolt ensued with an increasingly sectarian element. Well over 100,000 people have been killed and millions forced from their homes.
"We estimate that - from late 2011 to 10 December 2013 - between 3,300 and 11,000 individuals have gone to Syria to fight against the Assad government," said the International Centre for the Study of Radicalisation (ICSR), a partnership of five universities based at King's College London.
"These figures include those who are currently present (in Syria) as well as those who have since returned home, been arrested or killed."
The ICSR said it used 1,500 sources, including media reports, government estimates, jihadi statements and social media to collect the data.
The report showed that Arabs and Europeans made up the bulk of foreign fighters, with up to 80 percent, but militants from southeast Asia, North America, Africa, the Balkans and countries of the former Soviet Union were also represented.
Although only about 20 percent of the sources stated which rebel groups the foreigners joined, the ICSR said those sources indicated that most had fought for Nusra Front and the Islamic State of Iraq and the Levant, two groups linked to al Qaeda.
Western Europeans, with the largest contingents from France and Britain, represent up to 18 percent of the foreign fighters in Syria, it said. Up to 70 percent were from the Middle East.
It cited estimates of a three-fold increase of western Europeans from April this year, which it said may be linked to the deepening involvement in the war of Shi'ite Lebanese and Iraqi fighters, as well as of Shi'ite Iran, on the side of Assad, whose Alawite faith is an offshoot of Shi'ite Islam.
"This may have reinforced and strengthened the perception among some Sunnis that the conflict is fundamentally sectarian, and that Sunnis need to stand together in order to halt the (Shi'ite) enemy's advance," the report said.
The findings did not cover what are believed to be the thousands of foreign militants who have fought for Assad.
(Reporting by Oliver Holmes; Editing by Alistair Lyon)