A fierce battle broke out Saturday between Syrian rebels and the jihadist Islamic State of Iraq and the Levant (ISIL) around the northwestern town of Saraqeb, a monitoring group said.
"Fierce fighting took place around Saraqeb between ISIL and Islamist rebels vying for control of the town," said the Syrian Observatory for Human Rights.
The town fell out of President Bashar al-Assad's control in November 2012, and ISIL has been operational there since May last year.
On Saturday afternoon, a car bomb exploded south of Saraqeb, near an Islamist rebel checkpoint, said the Observatory.
Saraqeb is ISIL's last bastion in Idlib province, and rival rebels on Saturday launched an offensive to expel them from the town.
Syrian rebels fighting 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 northern 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.
In Raqa city, the only provincial capital to have fallen out of regime hands, ISIL took over a train station and a checkpoint previously under Islamist rebel control.
Dozens of corpses of ISIL fighters have been found in Raqa hospital, said the Observatory.
At the entrance to Jazra, a town west of Raqa, muezzins called for residents to go identify the abandoned bodies of dozens of Islamist rebels killed in fighting against ISIL, the group added.
Fighting between opposition groups in Syria and government forces has continued at the same time as the clashes in the north.
On Saturday, Alawite residents of Hama province who have relatives in Adra near Damascus reported the death in an explosion of seven women, four children and a man a day earlier, said the Observatory.
The group said rebel and government supporters have blamed each other for the blast in the flashpoint town.
The main opposition National Coalition meanwhile 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.