Syria's al-Qaeda affiliate lost ground to its jihadist rivals around a town on the Iraqi border on Thursday in heavy fighting that left 24 people dead, Agence France-Presse reported a monitoring group as saying.
Fighters of the Islamic State of Iraq and Syria (ISIS), a cross-border group which has been disowned by the al-Qaeda leadership, launched a three-pronged assault on positions held by al-Qaeda affiliate al-Nusra Front and its allies, the Syrian Observatory for Human Rights said, Al Arabiya reported.
ISIS fighters were driven out of Abu Kamal in heavy fighting earlier this year and are seeking to link up with their comrades over the border in Iraq, Observatory chief Rami Abdel Rahman told AFP.
"Since dawn there has been fierce fighting inside Abu Kamal. ISIS is advancing and has taken control of several neighbourhoods of the town which were previously held by Al-Nusra Front and Islamist brigades," he said.
A rebel commander loyal to the mainstream Free Syrian Army said it continued to control the nearby border crossing to the Iraqi town of al-Qaim and an AFP correspondent on the Iraqi side saw the FSA flag still flying over it.
"We are still controlling the border crossing," the rebel commander told AFP on condition of anonymity.
"ISIS are attacking our positions in Abu Kamal, and the battles since yesterday (Wednesday) are in three different areas about 15 kilometers (10 miles) from the center of the city. The battles are still continuing."
While ISIS was making gains, its head in Abu Kamal, Nader al-Rikhita, was killed during the clashes, Al Arabiya News Channel reported.
Abu Kamal has been under the control of fighters opposed to the Damascus regime since November 2012 but al-Nusra and its allies forced out ISIL fighters in heavy fighting earlier this year.
The Euphrates valley town had a pre-war population of some 70,000.
Syrian government troops control just one official crossing on the Iraqi border - Al-Tanaf/Al-Walid on the main highway to Baghdad.
A third crossing, Yarabiyah/Rabia in the northeast is controlled by Kurdish militia.
Washington backed away from military action in September after blaming President Bashar al-Assad for gassing civilians as it was hamstrung by fears over the dominance of the Nusra Front and ISIS in the rebels' ranks.
However, U.S. security sources recently told Reuters that Washington was finalizing a plan to increase training and small-arms shipments for Syrian rebels.
The United States would increase assistance and send the shipments to moderate rebel factions mostly based in Jordan, along Syria's southern border, the officials familiar with the plan told Reuters.