In a letter to The Times, Brigadier-General Abdulellah al-Basheer chief of staff of the supreme military council, the commanding body of the rebel Free Syrian Army (FSA), which opposes the Assad regime, said the "majority" of ISIL fighters were from the UK, with others from France, and Germany.
We, the Syrian people now experience beheadings, crucifixions, beatings, murders, outdated methods of treating women, an obsolete approach to governing society.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 general also called for the British government to send weapons to help deal with ISIL, warning that a failure to act would increase the threat of terrorism in Britain.
If ISIL is allowed to expand, these terrorists, having put their skills to the test in my country, will return to their homelands, perhaps to the UK, and continue on their pernicious path of destruction...
We are at a critical juncture in our fight against violent extremism and hope that the UK and US can shrug off their fear of supporting us.
A failure to actively support the FSA now will lead to ISIL's successes internationally.
More top news
Labour leader will put support for public ownership at the centre of the party's campaign for May's local elections in England.
Scientists are researching a blood test which can quickly detect colon, lung, breast, stomach and endometrial cancer.
Transport bosses are said to be in "active discussions" with the company to encourage it to extend its pilot scheme to the UK's capital.