Four British extremists have been killed fighting alongside al-Qaeda-linked fighters in Syria amid fears that jihadists returning from the war are a growing terrorist threat.

Three of the men, all thought to be from London, were killed as their group attacked pro-Assad forces near Aleppo. The fourth was shot two weeks later while trying to ambush an enemy position.

They were part of a group of ten 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.

MI5 estimates that 200 to 300 young Britons have travelled to fight in Syria, causing concern in security circles about the threat they pose when they return home.

A security source said: “Some of those