Al-Qaida-linked gunmen killed a rebel commander in Syria aligned with the Western-backed militias fighting against Bashar Assad's regime, the highest-profile casualty of growing tensions between moderate and jihadi fighters among rebel forces.

Observers worried Friday that the commander's death will increase distrust and suspicion between forces already at odds over territory and leadership as the nearly three-year civil war continues in Syria.

Loay al-Mikdad, a spokesman for the Free Syrian Army, said Friday that members of the Islamic State of Iraq and the Levant - a group reportedly made up of al-Qaida's branches in Iraq and Syria - were behind the killing of Kamal Hamami. Hamami, known as Abu Basir, served in the Supreme Military Council of the Free Syrian Army, a group headed by a secular-minded moderate that has the support of Western powers.

The Britain-based Syrian Observatory for Human Rights said gunmen shot Hamami dead late Thursday after militants tried to remove a checkpoint he set up in the Jabal al-Turkoman mountain in the coastal province of Latakia. The observatory said two of his men were seriously wounded in the shooting.

Al-Mikdad told Al-Arabiya TV that Hamami "was assassinated at the hands of the forces of evil and crime at one of the checkpoints." He added that the group that killed Hamami "should hand over those who carried out this act to stand trial."

Activists monitoring the war previously reported occasional clashes between rebel groups and Islamic militants active in rebel-held areas, especially in the north where the opposition has control of a large part of the region.

There also has been infighting between Kurdish and Arab groups over control of territory captured from the government along the border with Turkey in the past year. That fighting subsided after a cease-fire agreement early this year.

Hamami's killing marks the first time a commander from the Supreme Military Council of the Free Syrian Army has been killed by rebel jihadists. His death underlines a deepening power struggle between moderate and extremist groups fighting in the Syrian civil war.

"It's hard to tell where things are going to. It could really go either way," said Charles Lister, an analyst at IHS Jane's Terrorism and Insurgency Center. "I personally don't think it's in either of the sides' long-term interest to spark an escalation."