Islamic State group supporters released a video Tuesday that allegedly showed the beheading of Free Syrian Army rebels. The brutal killing was intended as a message to "any Muslim even thinking about joining any U.S.-backed group looking to over-throw the Assad regime," the video states.

Masked militants calling themselves "Wilayat Furat in Iraq," or the State of the Euphrates, said in the 19-minute propaganda video that they captured the Free Syrian Army rebels fighters near the Syrian border city of Al-Bukamal with Iraq, according to media reports. Rebel groups and Turkey have been working together in recent weeks to push the militants also known as ISIS from northern border towns in Syria.

ISIS, which opposes the Assad regime in Syria, has used brutal beheading videos to help recruit supporters and grow its caliphate across Syria and Iraq starting in 2014. The latest video threatens other anti-Assad rebel groups. It states that ISIS supporters have pledged their "blood and lives" to defeat non-believers working with "Syrian-American forces."

"We reinforce our brothers in the Islamic State," the men say in the video. "To Allah belongs all praise and favor."

The Free Syrian Army is made up of a group of mostly 60,000 former Syrian military soldiers fighting to force out the authoritarian regime of Syrian President Bashar Assad. Some of its members received training from CIA agents. Many of the soldiers defected from the Syrian military. Local and international volunteers have also joined up.