A still image taken on July 20, 2016 from an undated video posted on social media shows a boy before he was beheaded, on the back of a truck being taunted by rebel fighters in what is said to be Syria. Social Media

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

Images of a fighter cutting off the small boy’s head with a knife matched some of the worst brutalities committed by the jihadist Islamic State group (ISIS), which has killed hundreds of captives in Syria and neighbouring 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 channelled from Turkey, including U.S.-made TOW missiles.

U.S. State Department spokesman Mark Toner said Washington was seeking more information on what he described as “an appalling report”.

“If we can prove that this was indeed what happened and this group was involved … it would give us pause about any assistance or any further involvement with this group,” he told reporters.

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 what happened. “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.