Britons make up most of the foreign members in Syria's most violent terror group, a senior rebel leader says.
In a letter to The Times, Brig-Gen Abdulellah al-Basheer of the Free Syrian Army asks for help in curbing the Islamic State of Iraq and the Levant.
He claims the group attacks opposition forces, not the Assad regime
UK fighters are involved in activities including beheadings, crucifixions and ill-treatment of women, he adds.
In his letter, General al-Basheer writes that ignoring the problem could lead to British extremists returning home to "continue on their pernicious path of destruction".
Last week father-of-two Mashudur Choudhury became the first person in the UK to be convicted of terrorist offences in connection with the conflict in Syria.
Using the example of Choudhury, General al-Basheer writes: "He is one of many. They are not freedom fighters. They are terrorists."
He says the majority of non-Syrian members of the "predominantly foreign" Islamic State of Iraq and the Levant, a Sunni Islamist group, are from Britain. But he claims it includes fighters from Germany and France.
'Increasingly brutal threat'
He says there are also members from a range of countries across the Middle East, Africa and the Gulf and they pose an "increasingly brutal threat".
He writes: "We the Syrian people now experience beheadings, crucifixions, beatings, murders, outdated methods of treating women, an obsolete approach to governing society."
"Many who participate in these activities are British.
"The UK and US governments must support us to defeat terrorism in Syria and prevent it from being exported to Europe and the US."
General al-Basheer also claims that the group (which he refers to as ISIL, though it is also known as ISIS) kidnaps Free Syrian Army fighters and targets civilian homes in the city of Raqqa, instead of a nearby regime-controlled air base.
Tens of thousands of people have been killed in the Syrian conflict and millions displaced.
An estimated 500 Europeans are now fighting in Syria, according to the EU's anti-terror chief.
A spokesman for the Foreign Office said: "The police and security services are actively working to detect and disrupt any terrorist threat from Syria and individuals who travel there.
"People who are thinking about travelling to Syria to engage in terrorist activity should be in no doubt that we will take the strongest possible action to protect our national security, including prosecuting those who break the law."