Fighters from an al-Qaeda-linked group in Syria have killed a leader of the Western- and Arab-backed Free Syrian Army after stopping him at a checkpoint, an FSA spokesman said, underlining growing rifts between Syrian opposition groups.

Kamal Hamami, a member of the FSA's Supreme Military Council, known as Abu Basir, was killed in the Turkmen mountains near the northern city of Latakia, spokesman Louay Meqdad told Al Jazeera on Friday.

Meqdad said the commander was killed after a heated debate with a local leader of the Islamic State of Iraq and the Levant in which the leader called the FSA "infidels".

Hamami's brother, who was travelling in the car with him, was also killed, the spokesman said. The brothers and a third man had been on a surveillance mission before a planned attack on government forces, Meqdad said.

A third man was allowed to leave to report the killings.

Another FSA spokesman, Qassem Saadeddine, said the group phoned him to admit the killing.

"[They said] that they will kill all of the Supreme Military Council," Saadeddine said from Syria.

'Revolution threatened'

Meqdad said the incident was very worrying.

"We hope that we resort to wisdom rather than fighting as the Syrian revolution will be threatened as whole if the Syrian rebels are fighting among themselves," he said.

But other FSA rebels said the killings were tantamount to an act of war.

"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 told Reuters news agency on condition of anonymity.

He said the al-Qaeda-linked fighters had warned the FSA that there was "no place" for them where Hamami was killed in Latakia province, a rural region of Syria bordering Turkey.

The FSA has been trying to build a network of logistics and reinforce its presence across Syria, as the US

administration pledged to send weapons to the group after it concluded that Syrian President Bashar al-Assad's forces had used chemical weapons against rebel fighters.

US congressional committees are holding up the plan because of fears that such deliveries will not be decisive and the arms might end up in the hands of groups opposed to Western interests, security sources have said.

While FSA units sometimes fight alongside groups with different ideologies such as the Islamist State, rivalries have increased and al-Qaeda-linked groups have been blamed for several assassinations of commanders of moderate rebel units.

Analysts say that divisions between Syria's rebel groups are partly to blame for giving Assad's forces the chance to regain the upper hand in the conflict.