In a chilling reminder that not all of Syria’s rebels fighting forces loyal to President Bashar al-Assad share the same goals, an Islamist group has killed a leader of the Free Syria Army, reports said Friday.

Members of the Islamic State of Iraq and the Levant, an umbrella insurgent group with ties to Al-Qaeda that is linked to the deaths of thousands in neighboring Iraq since its 2006 formation, killed Kamal Hamami, who is also known as Abu Basir al-Ladkani, a member of the FSA’s Supreme Military Council, as the two rebel sides met to outline battle plans on Thursday, according to the BBC.

The killing is the first high-profile assassination of a high-ranking member of a rebel group representing Syrian moderates by another rebel group seeking to replace Al-Assad’s regime with a Sunni Islamist state.

The conflict between the rebel groups comes as the U.S. is preparing to arm some insurgents in Syria after the White House claimed last month that Al-Assad’s regime crossed a line by using chemical weapons. However, details on how U.S. taxpayer-funded military support would be dispersed among the warring groups isn't clear.

ABC News reported Friday that Syrian rebels, who are outmatched against Syria’s arsenal of weapons and military hardware acquired from Russia, are getting antsy about delays in delivering badly needed weapons from the U.S., which has committed to sending small arms but not the more effective antitank and antiaircraft weapons the rebels need to challenge loyalist forces.

Hamami’s killing was met with pledges of retribution and could open a new dynamic of confrontation in Syria’s 28-month old civil war that has killed as many as 120,000 people. The FSA and Islamist rebels have coordinated tactical maneuvers in the past.

"We are going to wipe the floor with them,” a senior FSA rebel commander told Reuters on Friday. “We will not let them get away with it because they want to target us."

The killing may have been sparked by a dispute over which side would control a key checkpoint in the northwestern port city of Latakia. Foreign Islamists have been pouring into the Syrian battlefields from abroad, viewing the battle to oust Al-Assad as part of a larger schism between Shiites and Sunnis.

Last month, Spanish authorities shut down a small recruitment cell that had sent as many as 50 Spanish and Moroccan Islamist rebels to Syria, including one minor and another man who committed suicide by driving a car bomb into a Syrian army camp last year.