Ammon News - Al-Arabiya - Jihadists fighting Syrian rebels killed at least five opposition fighters in a bomb attack in Idlib province on Saturday as clashes between the two sides raged in the country's north, activists said.

"Five Islamist rebels were killed in a bomb attack targeting their vehicle... in Saraqeb," the Syrian Observatory for Human Rights monitoring group said.

Activist Manhal Barish said an explosive device had been planted under a pick-up truck used by Ahrar al-Sham, an Islamist brigade that has led a week-long battle against the jihadist Islamic State of Iraq and the Levant.

The attack came as rebel fighters prepared for an assault on Saraqeb, ISIL's last bastion in the northwestern province of Idlib.

Syrian rebels fighting President Bashar al-Assad's regime initially welcomed the jihadists, but ISIL's abuses and their quest for hegemony turned the opposition against the group, which is rooted in Al-Qaeda in Iraq.

In the eastern city of Raqa, battles between jihadists and rebels raged as ISIL seized a border town near Tal Abyad, a key frontier post leading into Turkey, said the Britain-based Observatory, which uses a network of activists for its reports.

While ISIL has been on the defensive in Idlib and Aleppo provinces, where it was operational but not in control, its troops in Raqa are advancing against rebels.

Raqa is the only provincial capital to have fallen out of regime control.

Fighting between opposition groups in Syria and government forces has continued at the same time as the clashes in the north.

The main opposition National Coalition offered its "condolences" to the families of scores of rebels killed in an army ambush on Thursday as they tried to break a regime siege on part of Homs in central Syria.

"The Syrian Coalition commends the (rebel) Free Syrian Army fighters for their bravery, defending the Syrian people against the most brutal regime of the 21st century," the Coalition said in a statement.

Rebel-held parts of Homs city, once dubbed "the capital of the revolution" against Assad, have been besieged by the army for nearly 600 days.

On Thursday, the Observatory had reported 45 rebels killed as they tried to break out of Homs.

Activists in rebel districts regularly report severe food and medical shortages. International humanitarian organisations have been unable to negotiate entrance into the opposition-controlled districts.

"The hunger and total absence of any help from outside the besieged districts... led the (rebel) fighters to take suicidal measures to try and bring in some food and to open a route out for the families and the wounded," said a Homs-based activist who identified himself as Yazan.

Among the opposition fighters killed were the two brothers of Abdel Basset al-Sarut, a former football star-turned-rebel commander.

More than 130,000 people have been killed in Syria and millions more displaced since the conflict erupted in March 2011.