Members of the al - Qaeda - linked al - Nusra Front have killed a commander of the so - called Free Syrian Army, as rivalries grow among the militant groups fighting against the Syrian government.A video footage posted online shows al-Nusra militants killing an FSA commander execution-style. The authenticity of the video has not been verified. It is the second time in less than a month that al-Qaeda-linked militants kill FSA commanders. On July 11, members of the terrorist group, Islamic State of Iraq and Syria (ISIS), assassinated top FSA commander, Kamal Hamami, in the western city of Latakia. He was one of the top 30 figures in the FSA’s Supreme Military Command. Opposition sources said the killing followed a dispute between Hamami’s forces and the ISIS over control of a strategic checkpoint in Latakia as they were discussing battle plans. FSA spokesman Qassem Saadeddine has said that Takfiri militants have threatened to kill all senior commanders of the foreign-backed armed group. Al-Qaeda-linked groups and FSA have fought deadly battles across Syria several times since the beginning of the unrest in the Arab country more than two years ago. On June 5, dozens of militants were killed and wounded as violent infighting broke out between several Takfiri groups in Syria’s northwestern province of Idlib. Al-Nusra Front is also at loggerheads with Kurdish groups in northeastern Syria. Syria has been gripped by deadly unrest since 2011. According to reports, the Western powers and their regional allies -- especially Qatar, Saudi Arabia, and Turkey -- are supporting the militants operating inside Syria. According to the United Nations, more than 100,000 people have been killed and a total of 7.8 million of others displaced due to the violence.