BEIRUT (Reuters) - Syrian Islamist rebel group Ahrar al-Sham said on Thursday six other rebel factions had joined its ranks in northwestern Syria in order to fend off a major assault by a powerful jihadist group.

FILE PHOTO - A rebel fighter from the Ahrar al-Sham Islamic Movement reacts as they fire grad rockets from Idlib countryside, towards forces loyal to Syria's President Bashar al-Assad stationed at Jureen town in al-Ghab plain in the Hama countryside, April 25, 2015. REUTERS/Mohamad Bayoush

The hardline Jabhat Fateh al-Sham, once allied with al Qaeda and formerly known as the Nusra Front, attacked Free Syrian Army (FSA) groups west of Aleppo this week. It accused them of conspiring against it at peace talks in Kazakhstan.

Ahrar al-Sham, which presents itself as a mainstream Sunni Islamist group, sided with the FSA groups and said Fateh al-Sham had rejected mediation attempts. It said that any attack on its members of was tantamount to a “declaration of war”.

Rebel factions Alwiyat Suqour al-Sham, Fastaqim, Jaish al-Islam’s Idlib branch, Jaish al-Mujahideen and al-Jabha al-Shamiya’s west Aleppo branch said in a statement they had joined Ahrar al-Sham.

The Ahrar al-Sham statement also mentioned a sixth group, the Sham Revolutionary Brigades, and said “other brigades” had joined.

Ahrar al-Sham is considered a terrorist group by Moscow and did not attend the Russian-backed Astana peace talks. But it said it would support FSA factions that took part if they secured a favorable outcome for the opposition.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said Fateh al-Sham clashed with fighters from Ahrar al-Sham and Suqour al-Sham on Thursday in the northern rebel stronghold of Idlib province.

The attack by Fateh al-Sham had threatened to wipe out the FSA groups which have received backing from countries opposed to Syrian President Bashar al-Assad such as the Gulf Arab states, Turkey and the United States.

Internationally viewed as a terrorist group, Fateh al-Sham has been excluded from all diplomatic efforts to end the Syrian conflict, including a recent truce brokered by Russia and Turkey. Since the new year, the group has been targeted by a U.S. air strikes.

While Fateh al-Sham has often fought in close proximity to FSA rebels against Assad, it also has a record of crushing foreign-backed FSA groups during Syria’s complex, almost six-year conflict.