The Syrian Free Army says the assassination of one of its top commander by a rival rebel group linked to al-Qaeda is tantamount to a declaration of war.
The announcement is the latest sign of disarray among the opposition to President Bashar al-Assad.
The president has regained the upper hand in the insurgency which began more than two years ago.
Rivalry has been growing between the Western and Arab-backed FSA and al-Qaeda groups, who have sometimes joined forces on the battlefield.
Members of the Islamic State of Iraq and the Levant, a hardline group, killed Kamal Hamami of the FSA Supreme Military Council on Thursday. He was one of the FSA's top 30 figures.
Rebel commanders pledged to retaliate.
The BBC reports the Free Syrian Army was formed in 2011 by army deserters based in Turkey. It is said to have about 40-thousand members.