After months of fighting ISIS, al Qaeda's official Syrian arm the Al-Nusra Front "pledged loyalty to ISIS" in Albu Kamal, said Observatory director Rami Abdel Rahman.
"The pledge comes amid advances by ISIS in Deir Ezzor province" in eastern Syria on the Iraqi border, Abdel Rahman told AFP.
Although both ISIS and the Al-Nusra Front are rooted in al Qaeda, the two have been rivals for much of the time that ISIS has been involved in Syria's civil war since spring last year.
"They are rivals, but both groups are jihadist and extremists. This move will create tension now with other rebel groups, including Islamists, in the area," said Abdel Rahman.
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Meanwhile, the Syrian air force carried out air raids targeting ISIS-controlled Raqa in the north of the country and Muhassen in the east.
President Bashar al-Assad's regime has rarely targeted ISIS bastions, except in recent days after the group and other Sunni militants launched an offensive in Iraq, wresting control of Mosul and other pars of Iraq.
ISIS aspires to create an Islamic state that straddles Iraq and Syria.