BEIRUT (Reuters) - 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 the jihadist Islamic State group, 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. “We do not support groups that condone this sort of barbarity, period. We note that al-Zinki has said they will investigate this incident, and we hope they do so thoroughly and transparently.”

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.

It said it had detained the individuals who were involved in the incident and formed a committee to investigate. “All individuals who undertook the violation have been detained and turned over to the committee for investigations in accordance with the relevant legal standards.”

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.

“The child was ill, evidenced by the presence of a drip in his hand,” the statement said. The child in the video appears to have bandages round his waist and one of his legs, and a plastic medical tube near his arm.

The Syrian Foreign Ministry said it had written to the secretary-general of the United Nations asking him to condemn all crimes towards the Syrian people and Palestinian people in Syria. The letter asked the U.N. to punish states which support rebel groups in Syria.