Fighters of the mainstream rebel Free Syrian Army on Saturday battled jihadists from an Al-Qaeda affiliated group who tried to seize their weapons in the northwest, a monitoring group said.
Tensions have been mounting between the FSA and the jihadist Islamic State of Iraq and Syria and boiled over after ISIS fighters shot dead an FSA commander two days ago.
Saturday's clashes erupted at dawn near Ras al-Hosn, in the north of Idlib province, when "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.
Mainstream rebels fighters and jihadist groups such as ISIS and Al-Nusra Front, whose ranks include non-Syrian fighters, have clashed repeatedly in recent weeks.
On Thursday, ISIS fighters killed FSA commander Kamal Hamami -- better known by his nom-de-guerre Abu Bassir al-Jeblawi -- at a checkpoint in the coastal province of Latakia, an incident that is expected to further rachet up tensions.
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.
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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 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.
In other violence on Saturday, rebels and regime forces fought pitched battles as regular troops tried to re-open a highway between Latakia and the northern city of Aleppo much needed as a supply route for the military.
"The regime is trying to channel foodstuff to neighbourhoods of western Aleppo which are under its control," the Observatory's Abdel Rahman said.
Rebels have been besieging these districts also to stop the army from receiving military supplies, the Britain-based Observatory said.
Meanwhile Aleppo residents protested for the second time this week against the rebel siege of regime-held districts in Syria's second city, where food shortages are mounting, the Observatory said.
Rebel forces entered Aleppo a year ago and have seized large swathes of the northern city but have failed to capture all of it.
The Observatory said a protester was killed on Tuesday in Aleppo when activists demonstrated against sieges by rebels on areas controlled by the army where troops are unable to deliver food.