Syria's Al-Qaeda affiliate lost ground to its jihadist rivals around a town on the Iraqi border Thursday in fierce clashes that left at least 51 fighters dead, a monitoring group said.

Al-Nusra, the affiliate of Al-Qaeda, meanwhile, claimed responsibility for a twin car bombing in a pro-government area of the central city of Homs that killed 25 people on Wednesday.

Fighters of the Islamic State of Iraq and the Levant (ISIL), a cross-border group disowned by Al-Qaeda's leadership, launched a three-pronged assault on positions held by Al-Nusra and its allies, the Syrian Observatory for Human Rights said.

At least 51 jihadist and Islamist fighters were killed, the monitoring group said, updating an earlier toll but without giving a breakdown.

The clashes centred around the border town of Albu Kamal, from which ISIL was expelled earlier this year, said the Observatory.

The bodies of another 10 fighters, apparently executed by ISIL, were found at two sites near Albu Kamal, it said, adding there were reports of other summary executions in the same area.

Earlier, the monitoring group's head Rami Abdel Rahman told AFP that ISIL was advancing in Albu Kamal and had taken control of several districts previously held by Al-Nusra and other Islamist brigades.

The clashes prompted soldiers on the Iraqi side of the border to reinforce their positions.

The border crossing itself on Syria's side remained in the hands of the mainstream rebel Free Syrian Army, according to a rebel chief and an AFP reporter on the Iraqi side who saw the FSA flag flying over it.

ISIL fighters are seeking to link up with their comrades over the border in Iraq, said Abdel Rahman.

- Iraq blast walls -

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 district of Karm al-Loz in 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 its devastated Old City.

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