The local unit of Al-Qaeda's Syrian branch in the tinderbox town of Albu Kamal on the Iraqi border pledged loyalty Wednesday to the Islamic State of Iraq and the Levant, giving ISIL control over both sides of the frontier.
The move is also significant because it reflects how ISIL is fast gaining the upper hand in eastern Syria, where it has been locked in combat with fighters from Al-Qaeda franchise Al-Nusra Front and allied local rebels virtually all year.
Al-Nusra's oath of loyalty in Albu Kamal comes days after Iraqi security forces abandoned Al-Qaim, just across the border, and ISIL and other Sunni militants seized it on Saturday.
ISIL, which aspires to create an Islamic state that straddles Iraq and Syria, has spearheaded a lightning jihadist offensive that has already captured swathes of territory north and west of the capital.
ISIL waded in to Syria's civil war in the spring of last year on the side of rebels seeking to oust President Bashar al-Assad, but its systematic abuses and quest for hegemony quickly turned Syrian rebels, including Islamists, against it.
As a result, fighting broke out in January between ISIL and Syrian rebels, which eventually drew Al-Nusra in against its fellow jihadist organisation.
Despite the fighting, which has killed hundreds, activists say the offensive in Iraq has empowered the group, partly because its fighters have captured large amounts of heavy weaponry from fleeing Iraqi troops.
On Wednesday, Al-Nusra's Albu Kamal branch "pledged loyalty to ISIL," said Observatory director Rami Abdel Rahman.
"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.
An ISIL jihadist confirmed the reports on Twitter, posting a photo showing an Egyptian Al-Nusra Front commander shaking hands with an ISIL leader of Chechen origin.
An opposition activist in Albu Kamal told AFP via the Internet that "there is a lot of tension, and the situation is only going to get worse."
- 'Catastrophe' threatens town -
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Using a pseudonym for security reasons, Hadi Salameh also said the merger would "cause a big problem with the local tribes, who will not welcome this change."
Another activist said the move comes days after local rebel brigades who had been working with Al-Nusra signed a declaration demanding that it take a clear stance against ISIL.
"The loyalty oath (to ISIL) comes after tension between Al-Nusra and the local rebels," said Abdel Salam al-Hussein.
He also said hundreds of thousands of people, including displaced families from neighbouring Iraq as well as flashpoint areas in Syria, are living in Albu Kamal, and that it would be a "catastrophe" if fighting broke out in the town.
Hussein said: "ISIL fighters are now positioned at the entrance of Albu Kamal, on the Iraqi side."
Meanwhile, Deir Ezzor province's rebel spokesman Omar Abu Leyla warned that "Albu Kamal is a red line." If ISIL fighters cross over from Iraq, he said the opposition "Free Syrian Army will fight them."
Rebels fighting ISIL and the regime of Syrian President Bashar al-Assad distributed amateur video footage of a rebel parade in Albu Kamal, which Abu Leyla described as a warning to the jihadists positioned just across the border.
Abu Leyla complained that "the FSA has received no external support at all, even though we are fighting ISIL."
Separately Wednesday, the Syrian air force raided ISIL-controlled Raqa in the north of the country and Muhassen in the east.
In Raqa, "12 civilians, including a woman and a child, were killed in the air strikes. Not one strike directly hit an ISIL position."
The Assad regime has rarely targeted ISIL-held areas, 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.
A Syrian government newspaper reiterated frequent regime claims that the United States and Israel are behind the rising violence, and that they are vying to "divide Syria along sectarian and religious lines."