A senior rebel commander in Syria is reported to have been killed by rebels from a rival group linked to al-Qaeda.
Kamal Hamami, of the Free Syrian Army's (FSA) Supreme Military Council, was meeting members of the rival group "to discuss battle plans".
An FSA spokesman said he was told by the Islamic State of Iraq and the Levant that they had killed Mr Hamami.
The killing is part of an escalating struggle within the armed uprising between moderates and Islamists.
The BBC's Paul Wood says a civil war within a civil war is building within the opposition as the two sides engage in a battle that is partly over the spoils and partly ideological.
Kamal Hamami, also known as Abu Basir al-Ladkani, worked in a butcher's shop before the conflict began, but went on to form one of the FSA's key brigades in his native port city of Latakia.
He is believed to have met members of the Islamic State of Iraq and the Levant in Latakia to inform them of a planned offensive in the area, before being ambushed and shot dead.
A spokesman for the FSA's Supreme Military Council said Mr Hamami had been driving through Latakia when he and fellow fighters encountered a checkpoint run by the Islamic State of Iraq and the Levant - the main group linked to al-Qaeda.
The rebels manning the checkpoint had refused to let him pass, saying he would need to get permission from their leader, and he had told them they had to take their checkpoint down.
As the argument raged, according to this account, one of the fighters - said to be a foreign jihadi from Iraq - raised his weapon and shot Kamal Hamami dead.
"The Islamic State phoned me saying that they killed Abu Basir and that they will kill all of the Supreme Military Council," FSA spokesman Qassem Saadeddine told Reuters news agency.
The FSA was established in 2011 by army deserters based in Turkey and is said to have some 40,000 members.
Although the FSA have had some successes in the fight against President Assad's forces, they say they will be unable to win the war unless they acquire more sophisticated weaponry.
In recent months, Western and Arab nations have agreed to step up support for moderate Syrian rebels in their battle against President Bashar al-Assad's forces.
But many in the West are nervous about heavy weaponry falling into the hands of radical groups.
Al-Qaeda in Iraq announced in April that it was merging with the Syrian Islamist group, the Nusra Front, to form the single Islamic State of Iraq and al-Sham (the Levant).
The Nusra Front - which had gained a reputation for discipline and honesty - rejected the merger though not its allegiance to al-Qaeda.
The leader of al-Qaeda, Ayman al-Zawahiri, has urged its fighters to strive for an Islamic state in Syria.
The spread of Sharia in rebel-held areas has alarmed the moderate members of the opposition, who, though Muslim, do not want a religious state.