Militants take a photo of a boy before beheading him

DAMASCUS — A Syrian rebel group that has received U.S. military backing said it is investigating the beheading of a child in Aleppo after video footage circulated showing the boy being killed by a man who activists identified as a member of the rebel group.

Images of a fighter cutting off the boy’s head with a knife matched some of the worst brutalities committed by ISIS, which has killed hundreds of captives in Syria and neighboring Iraq in the last three years.

Before being killed, the boy is shown on the back of a truck being taunted by several men who say he is from a Palestinian faction which fights in Aleppo in support of President Bashar al-Assad.

“This is a prisoner from the Quds Brigade. They don’t have men any more so they’ve sent us children today,” one of the men says. “These are your dogs, Bashar, children of the Quds brigade,” says another.

The Syrian Observatory for Human Rights said the men were fighters from the Nour al-Din al-Zinki Movement, a rebel group which has received military support channeled from Turkey, including U.S.-made TOW missiles.

The U.S. State Department said on Tuesday that it was investigating the incident.

On Wednesday, a State Department official said the United States does not publicly identify the Syrian opposition groups that receive U.S. support.

“We routinely vet the groups we work with and support, as you might expect, and their human rights record figures prominently in that,” said the State Department official, who declined to be identified.

In a statement, Nour al-Din al-Zinki denounced what it described as “the human rights abuses that were shared on social media sites,” which did not represent its policies or practices.In an online statement, the Quds Brigade denied the 12-year-old child was a fighter. It said he was from a poor family living in the Handarat Camp area north-east of Aleppo, which had fallen under the control of rebels.