Human rights campaigners have warned that Syrian opposition fighters are conducting summary killings, using child soldiers, sectarian violence and committing war crimes, as well as their pro-government foes, cautioning governments about arming the rebel groups.

The Amnesty International report, released this month on the second anniversary of the conflict, comes as David Cameron hinted for the first time that Britain could be prepared to go it alone to supply arms to Syria's rebels, defying a European arms embargo.

He, and the Foreign Secretary William Hague, have stressed the need to identify less-radical groups to combat the worrying rise of Islamic fundamentalism in Syria.