A new video posted online showing US-backed Syrian terrorists brutally beheading a child has caused public outrage.

The footage, which circulated on social media on Tuesday, showed a member of the Nureddin al-Zenki opposition militant group, which is supported by the US, cutting off the small boy's head with a knife on a public road in Aleppo's opposition-controlled al-Mashhad neighborhood.

Before being killed, the boy, who could be as young as 10, is shown on the back of a truck being taunted by militants, who accuse him of being a member of al-Quds Brigades, the armed wing of the Palestinian Islamic Jihad resistance movement, which fights alongside the Syrian government forces.

However, the Palestinian group issued a statement, denying the boy was a member, but rather identifying him as a 12-year-old Palestinian refugee.

Local residents denounced the beheading, saying it tarnished the image of the Syria opposition.

"How could they slaughter a child like this?" said 25-year-old Bassel Zein, a barber in the opposition-held al-Kalasseh neighborhood of Aleppo, adding, "He should have been tried in a fair way” and maybe exchanged in a prisoner swap “instead of this heinous act.”

An Aleppo mosque imam, Mohamed Badawi, also described the execution, which is forbidden in the holy faith of Islam, as “the work of criminals”.

Nureddin al-Zenki militant group, which is largely based in Syria’s Aleppo Province, has received military support channeled from Turkey, including US-made anti-tank missiles.

Earlier this month, Amnesty International said the militant group was among several opposition outfits committing war crimes and other violations of international law.

Syria has been gripped by foreign-sponsored militancy since March 2011.

The Daesh terrorists have been carrying out horrific acts of violence, such as public decapitations and crucifixions, against all communities, including Shias, Sunnis, Kurds, and Christians in areas they have overrun in the Arab country.

The Takfiri elements have suffered major setbacks over the past few months as the Syrian army forces have managed to liberate a number of areas from the grip of the extremists.