Ex-Qaeda in Syria joins forces with radical group
The announcement, on Fateh al-Sham Front's Twitter account, came as Jund al-Aqsa fighters are locked in fierce clashes with other rebel groups in northern Syria's Idlib province.
"In a bid to avoid shedding the blood of Muslims and overcome the internal fighting between us and Ahrar al-Sham, which only profits the regime, we, Jund al-Aqsa announce that we have pledged allegiance to Fateh al-Sham Front," said the online statement.
The statement was handwritten and bore the signatures of the heads of both groups.
The powerful Ahrar al-Sham group which has close ties to Fateh al-Sham Front has accused several members of Jund al-Asqa of links to the Islamic State group.
IS is the key rival of Fateh al-Sham Front, formerly known as al-Nusra Front and al-Qaeda's affiliate on the battlegrounds of Syria.
Analysts said that Sunday's announcement was expected to complicate things for Fateh al-Sham Front.