MI5 is also saying the first wave of jihadists who left the UK for Syria have already returned.

(AFP) – Four British men have been killed fighting alongside Al-Qaeda-linked fighters in Syria, it was reported Thursday, as fears rise of the growing threat from jihadists returning from the war.

Three of the men, all thought to be from London, were killed in August as their group attacked forces loyal to President Bashir al-Assad near Aleppo, according to The Times.

The fourth man was shot two weeks later while trying to ambush an enemy position.

The Times said they were part of a ten-strong group of British jihadists who fought together and joined up with 20 other Britons to fight alongside the Al-Nusra Front, which is allied to Al-Qaeda.

Britain’s intelligence agency MI5 estimates that between 200 and 300 young Britons have travelled to fight in Syria, and are concerned they will recruit new converts to their cause, or launch attacks at home.

A security source told The Times: “Some of those who went out in the early stages of the conflict have been able to return to Britain, radicalise others, and go back to Syria with others in tow.”