Al-Qaeda's official offshoot in Syria has pledged allegiance to Isis at a flashpoint town on the Iraqi border, according to a monitoring group.
The British-based Syrian Observatory for Human Rights claims the merger between the two rival groups will allow Isis - an acronym for the Islamic State of Iraq and the Levant - to control both sides of the border: Albu Kamal in Syria and Al-Qaim in Iraq.
Al-Nusra Front, the official Syrian franchise for the global terror network, has "pledged loyalty to Isis", according to the Observatory.
"The pledge comes amid advances by Isis in Deir Ezzor province," in eastern Syria, the group told AFP.
A jihadist twitter account confirmed the reports and posted a picture showing an Egyptian al-Nusra commander shaking hands with Omar al-Shishani, the Isis leader of Chechen origin.
The two jihadist groups both have al-Qaeda links but have been fighting each other for months, since Isis became involved in the civil war.
An activist explained that the merger took place days after local rebel brigades, who had been working with the al-Nusra front, signed a declaration excluding the al-Qaeda branch from the Islamic court.
Al-Qaeda cut ties with Isis in 2013 in what was believed to be a bid to reassert influence among rival Islamic groups in Syria.
The jihadist group led by Abu Bakr al-Baghdadi reportedly disobeyed orders from network leader Ayman al-Zawahri not to operate independently from al-Nusra in the country. Baghdadi reportedly dismissed Zawahri's orders and attempted to merge the two branches.
Isis and al-Nusra emerged as the two main militant Islamic groups in Syria. Over time, Isis eclipsed al-Nusra in many areas in the north of the country.
As opposed to fighters from Al-Qaeda's official offshoot in Syria, the al-Nusra front, members of Isis have been described by Syrian refugees as "foreign 'occupiers'" whose only goal is creating a caliphate, a proto-state that straddles Syria and Iraq.