"We will not let them get away with it because they want to target us," a senior FSA commander told Reuters.
The fighting between Islamic and secular opposition groups threatens to open up a new front in the two-year Syrian civil war and give Assad’s forces even more of an advantage.
It could also jeopardize military aid that the United States has said it will send to the Syrian rebels to help them against Assad’s regime, which is being aided by Russia and Iran.
Congress has put a hold on the Obama administration’s plans to provide military aid to the rebels, out of fear that the weapons will end up in the hands of al Qaeda-linked groups.
The administration is currently working with the Intelligence Committees to address their concerns. Administration officials have said that they will only provide weapons to vetted rebel groups.
Thursday’s killing of Hamami, known also as Abu Baseer al-Ladkani, occurred in the Latakia province on the northwest coast of Syria.
The Observatory said Hamami was killed when ISIS tried to take down a checkpoint set up by the Free Syrian Army.
“Gunfire was then fired by the [FSA] battalion members into the air, an ISIS rebel then fired on Abu Baseer and 2 members of his battalion, who were badly injured,” the Observatory wrote.