The assassination of a top Free Syrian Army commander by militants linked to al Qaeda is tantamount to a declaration of war, FSA rebels on Friday, opening a new front between Western-backed forces and Islamists in Syria's civil war.
The announcement is the latest sign of disarray in the armed opposition to Syrian President Bashar al-Assad, who has regained the upper hand more than two years into an insurgency that grew out of Arab Spring-inspired pro-democracy protests.
It follows growing rivalries between the FSA and the Islamists, who have sometimes joined forces on the battlefield, and coincides with attempts by the Western and Arab-backed FSA to allay fears any US-supplied arms might reach al Qaeda.
Members of the Islamic State of Iraq and the Levant, a hardline Islamist group, killed Kamal Hamami of the FSA Supreme Military Council on Thursday. Also known by his nom de guerre, Abu Bassir al-Ladkani, he is one of its top 30 figures.
Rebel commanders pledged to retaliate.
"We are going to wipe the floor with them. We will not let them get away with it because they want to target us," a senior rebel commander said on condition of anonymity.
He said the al Qaeda-linked militants had warned FSA rebels that there was "no place" for them where Hamami was killed in Latakia province, a northern rural region of Syria bordering Turkey where Islamist groups are powerful.
Other opposition sources said the killing followed a dispute between Hamami's forces and the Islamic State over control of a strategic checkpoint in Latakia and would lead to fighting.
The FSA has been trying to build a logistics network and reinforce its presence across Syria as the US administration considers sending weapons to the group after concluding that Assad's forces had used chemical weapons against rebel fighters.
The anti-Assad Syrian Observatory for Human Rights, which monitors the conflict, said the FSA and the Islamic State have had violent exchanges in several areas of Syria over the past few weeks, showing growing antagonism between Assad's foes.
"Last Friday, the Islamic State killed an FSA rebel in Idlib province and cut his head off. There have been attacks in many provinces," the Observatory's leader Rami Abdelrahman said.
Two of Hamami's men were wounded in Thursday's attack, he said by telephone.
Syria's conflict turned violent in the face of a crackdown on protests. Civil war ensued with disparate rebel groups taking up arms and the Observatory says more than 100,000 people have been killed.
US congressional committees are holding up plans to arm the rebels because of fears that such deliveries will not be decisive and the arms might end up in the hands of Islamist militants.
Syria's opposition bemoans the delay, and repeated on Thursday assurances that the arms will not go to Islamist militants.