Jihadists from the Islamic State of Iraq and the Levant bolstered their presence Thursday in the Syrian town of Albu Kamal on the border with Iraq, a monitoring group said.
The Syrian Observatory for Human Rights said the move came a day after members in the town of Al-Qaeda's franchise in Syria, Al-Nusra Front, pledged loyalty to ISIL, giving it control over both sides of the frontier.
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 Baghdad.
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Pro-ISIL militants posted videos on YouTube showing a convoy of cars and trucks entering Albu Kamal and flying the jihadists' black flag.
Other supporters announced the reinforcement on Twitter, with one saying: "ISIL convoys enter Albu Kamal after conquering it, thanks to Allah."
A statement from Al-Nusra released in the town said the Al-Qaeda franchise pledged loyalty to ISIL in Albu Kamal to prevent bloodshed between the two rival jihadist groups.
Albu Kamal is located in Syria's oil-rich eastern province of Deir Ezzor, where ISIL and Al-Nusra, who have been fighting to topple the Syrian regime, have also been fighting each other since January.
Romain Caillet, an expert on Islamists groups, said Al-Nusra fighters pledged allegiance to ISIL in the border town because "it is powerful, they fear it and they have been weakened on the ground".