Syria conflict: Who are ISIS?
9 January 2014 Last updated at 10:15 GMT
Recent days have seen fierce fighting between the al-Qaeda-affiliated Islamic State in Iraq and the Levant (ISIS) and other rebel groups in Syria.
On Thursday, Islamist rebels captured the ISIS headquarters in Aleppo.
More than 270 people, including 46 civilians, have been killed in the worst rebel-on-rebel violence since the uprising in Syria began in March 2011, according to the Syrian Observatory, a UK-based activist group.
James Reynolds reports from the Turkey-Syria border on Syria's "second war".