A group of around 1,500 Chechen, Uzbek, and Tajik fighters in Syria has pledged allegiance to Al-Qaeda's Syria wing Nusra Front, a group monitoring the war said September 23.

Jaish al-Muhajireen wal-Ansar (Muhajireen Brigade) made the pledge in a statement distributed by supporters online, the Syrian Observatory for Human Rights said.

The pledge is a boost for Nusra Front against its rival Islamic State, an ultra hardline Sunni Muslim group which has seized territory in Syria and Iraq.

Among battles fought between Nusra Front and Islamic State is one for control of parts of the northern Syrian city of Aleppo, where fighters from the Muhajireen Brigade could help tip the balance.

Nusra Front, loyal to the successors of Osama bin Laden, and Islamic State are the two most powerful forces fighting the Syrian army and its allies in Syria's civil war. The groups have fought each other since a split in 2013 largely due to a power struggle between leaders.

Nusra Front's leader said in June his group had around 30 percent foreign fighters, including Europeans, Asians, Russians, and Chechens.

Based on reporting by Reuters and AFP