Jihadists and members of hardline Islamist groups make up almost half of forces fighting against Syrian President Bashar al-Assad, according to extracts from a British defence study published in Monday's Daily Telegraph.
The analysis by defence consultancy IHS Jane's, due to be published in full later this week, puts the number of rebel forces at around 100,000, the Telegraph reported.
But these fighters have split into as many as 1,000 bands since violence flared two years ago, the study concluded.
Of the rebel forces, IHS Jane's estimates that around 10,000 are jihadists fighting for groups linked to Al-Qaeda and another 30,000 to 35,000 are hardline Islamists, who differ from jihadists in that they are concentrated only on the Syrian conflict, and not on the global Islamist fight.
"The insurgency is now dominated by groups which have at least an Islamist viewpoint on the conflict," Charles Lister, author of the analysis, told the British newspaper.
"The idea that it is mostly secular groups leading the opposition is just not borne out.
"If the West looks as though it is not interested in removing Assad, moderate Islamists are also likely to be pushed further towards extremists," he warned.
The study is based on interviews with militants and on intelligence estimates.