Hardline Islamists and jihadists linked with al-Qaeda make up almost half of 100,000 fighters fighting against Syrian President Bashar al-Assad, according to a new defence study.
"Nearly half the rebel fighters in Syria are now aligned to jihadist or hardline Islamist groups," the daily Telegraph quoted the British study as saying.
Citing extracts from the study to be published in full later this week, the paper said that forces battling the Assad regime now number around 100,000 fighters and these fighters have fragmented into as many as 1,000 groups.
The study by defence consultancy IHS Jane's estimates there are around 10,000 jihadists - including foreign fighters - linked to al-Qaeda and another 30,000 to 35,000 are hardline Islamists who share much of the outlook of the jihadists.
There are further 30,000 moderates belonging to groups that have an Islamic character, meaning only a small minority of the rebels are linked to secular or nationalist outfits.
"The insurgency is now dominated by groups which have at least an Islamist viewpoint on the conflict. The idea that it is mostly secular groups leading the opposition is just not borne out," said Charles Lister, author of the analysis.
"Two factions linked to al-Qaeda, Jabhat al-Nusra and the Islamic State in Iraq and the Levant (ISIL) - also know as the Islamic State of Iraq and al-Shams (ISIS) - have come to dominate among the more extremist fighters," he said.
Their influence has risen significantly in the past year. Being better armed and tougher fighters, ISIL and al-Nusra have taken control of much of the income-generating resources in the north of the country, including oil, gas and grain.
The new assessment contradicts US Secretary of State John Kerry who had recently said that only 15 per cent to 20 per cent were "bad guys" or extremists among rebels in Syria.
The aim of moderate fighters is the overthrow of the Assad regime, but jihadist groups want to transform Syria into a hard-line Islamic state within a regional Islamic "caliphate", the paper said.
"Al-Qaeda has assassinated several moderate Free Syrian Army rebel commanders in northern Latakia province in recent weeks, and locals say they fear this is part of a jihadist campaign to gain complete control of the territory," it said.
The study is based on intelligence estimates and interviews with activists and militants.
