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 31 people dead, a monitoring group said.
Fighters of the Islamic State of Iraq and the Levant (ISIL), a cross-border group which has been disowned by Al-Qaeda's 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-Nusra, meanwhile, claimed responsibility for a twin car bombing in a pro-government area of the central city of Homs that killed 25 people and wounded more than 100 on Wednesday.
ISIL fighters were driven out of Albu 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 Albu Kamal. ISIL 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 still controlled 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.
"ISIL are attacking our positions in Albu Kamal, and the battles since yesterday (Wednesday) are in three different areas about 15 kilometres (10 miles) from the centre of the city. The battles are still continuing."
By early evening, the Observatory said at least 31 fighters from both sides had been killed on Thursday.
Later in the day, Iraqi troops put up blast walls and sand berms on their side of the crossing, the AFP correspondent reported.
Albu 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.
Al-Nusra said Wednesday's twin bombing in the mainly Alawite Karm al-Loz district of Homs had targeted militia loyal to President Bashar al-Assad's Alawite-dominated regime.
In a statement posted on Twitter, the group said its militants had "penetrated one of the bastions of the Shabiha and placed two car bombs that detonated a half hour apart during a Shabiha gathering."
On Thursday, rebels, including Al-Nusra fighters, seized a building near a military intelligence headquarters in the Zahra district of the main northern city of Aleppo, the Observatory said.
In Homs, unidentified gunmen killed 14 people, including several children, in the Karm al-Zeitun neighbourhood.
The Observatory said the attack was carried out by pro-regime militia, but the state SANA news agency said an "armed terrorist group" was behind it.
Homs was an epicentre of the revolt but is now almost entirely in regime hands, with small pockets of rebels holding out in besieged areas in and around the devastated Old City.