Inter-opposition fighting, which has included killings, kidnappings and beheadings, has been ongoing in Syria between the Free Syrian Army and extremist groups with al-Qaida links, such as the Islamic State of Iraq and Al Sham, The New York Times reported.
Members of the Islamic State killed Kamal Hamami, the Free Syrian Army commander, in the coastal province of Latakia Thursday, after he had met with others in his group about getting weapons.
"The Islamic State wants to eliminate the Free Syrian Army higher command," Ahmed Farzat, a Free Syrian Army lieutenant, said in an interview from Homs. "In other words, to marginalize it and replace it."
The Islamic State has also been accused of beheading two Free Syrian Army fighters last week and leaving their heads beside a garbage can in a square in Dana.
The attack came after 13 people were killed in clashes at a demonstration against the Islamic State in Dana.
"We staged demonstrations to get freedom, not to have an emir ruling us," said Abu al-Haytham, a fighter from the area, referring to the title used by Islamist commanders.
"The sea is in front of us, and the enemy is behind us," said Sheik Jassem al-Awad, a tribal leader in Raqqa, adding he felt squeezed between the government and the radical Islamists. "The Free Syrian Army cannot open two fronts at once."
Awad said he and eight others were arrested by the Islamic State and held captive for 25 days, during which they were tortured.
Jamil Sello, who was held captive with Awad, said he suffered several broken ribs from beatings and had been accused of "trying to establish a secular state, collaborating with the U.S. intelligence and Qatar."