Rebel fighters battled the Al-Qaeda affiliated Islamic State of Iraq and Syria on Saturday after the jihadist group tried to seize their weapons, a monitoring group said.
The clashes erupted at dawn near Ras al-Hosn, in the north of Idlib province, when the "ISIS fighters tried to seize weapons stored in Free Syrian Army depots," Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, told AFP.
The fighting comes amid mounting tensions between mainstream rebel forces and radical Islamist groups including ISIS and the Al-Nusra Front.
Tensions have grown in recent months as opposition fighters and activists accuse the groups, which include among their ranks a number of non-Syrian fighters, of increasingly oppressive behaviour.
On Thursday, ISIS fighters shot dead a Free Syrian Army commander in the coastal province of Latakia, an incident that is expected to further ratchet up tensions.
Kamal Hamami -- better known by his nom-de-guerre Abu Bassir al-Jeblawi -- was killed at a checkpoint, though accounts of his death differed.
A rebel allied to Abu Bassir said via Facebook he had witnessed what he called a cold-blooded shooting at an ISIS checkpoint when the rebel chief was on his way to visit fellow fighters at the front.
The Observatory said he was killed when ISIS fighters tried to destroy an FSA checkpoint in the Jabal al-Turkman region, in the north of Latakia province.
Idlib province, in Syria's northwest has already seen clashes between mainstream rebels and jihadists.
Late last week, dozens of FSA fighters were killed in a battle against ISIS, according to the Observatory.
The FSA battalion chief there was beheaded by ISIS and his brother slaughtered, the group reported.
In the early days of the Syrian uprising, when opponents of the regime were desperate for assistance from any quarter, jihadist fighters were welcomed.
But activists and other rebels complain that jihadist groups are power-hungry and seek to impose austere religious practices on an observant but heterodox population.
They have also been angered by arbitrary detentions and killings by jihadists, including the execution of a 15-year-old boy accused of uttering a blasphemous phrase.