20/7/2016: A previous version of this article estimated the age of the boy at about 10 years old. His actual age, according to the Jerusalem brigade, was 12, as reflected in the altered headline.

A video circulating on social media appears to show Syrian rebels beheading a Palestinian boy who they accused of being a pro-government fighter.

The video, posted online on Tuesday, shows a group of men in the back of a truck cutting off the head of the terrified boy.

Before the gruesome murder, the men accuse the boy of being a member of the Liwa al-Quds (Jerusalem brigade), a Palestinian armed group fighting alongside Syrian President Bashar al-Assad's government.

The Jerusalem brigade posted a statement on its official Facebook page on Wednesday, identifying the boy as 12-year-old Abdullah Issa, who they said was not a fighter.

"He lived in al-Mashhad [Aleppo] with his family, among multiple poor families that live in the area under the control of terrorists," read the statement, adding that the boy was ill.

"By taking one glance at the child - the argument that he was a fighter is immediately disapproved," the group said.

In the video, one of the rebels shouts: "He sent us a child today, he no longer has men!" seemingly referring Assad.

"We will leave no one in Handarat," they added, referring to the city north of Aleppo where heavy fighting between rebels and government forces has been taking place.

The rebels have been identified as members of the Nour al-Din al-Zinki Movement. On Tuesday, the movement published a press release on its Twitter page condemning the killing of the boy and calling the beheading an "individual error that does not represent the general policy of the movement".

The United States, which has previously provided military support to the Nour al-Din al-Zinki Movement, said it was "seeking more information" and that it could not confirm the "appalling report" at a press briefing on Tuesday.

"If we can prove that this was indeed what happened and this group was involved in it, I think it would certainly give us pause," State Department Spokesman Mark Toner told reporters, after being asked whether this incident would affect US assistance to this specific group.

The UK-based Syrian Observatory for Human Rights monitoring group said the video shows fighters from rebel factions "slaughtering a boy member from the gunmen loyal to the regime".

"The boy was captured during the clashes at Handarat camp this morning," the Observatory said in a statement.

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A report published earlier this month by human rights group Amnesty International details several "war crimes and other violations of international humanitarian law" committed by the Nour al-Din al-Zinki Movement, as well as other rebel groups, throughout the Syrian Civil War.

The alleged war crimes include abductions and torture.

Citing sources close to the United States "northern operation command," a coordination body allegedly responsible for providing military support to rebels, Amnesty's report says the Nour al-Din al-Zinki Movement had in the past received military and financial aid from the US, Britain, France, Turkey and several Gulf states.

The funding was apparently halted in September 2015 when the movement joined two other groups that were accused of kidnapping Italian aid workers.

The death toll in the Syrian conflict, which began with mostly unarmed demonstrations against al-Assad in March 2011, has risen to more than 280,000 people, while half the country's population have been forced from their homes, according to United Nations estimates.

Staffan de Mistura, UN special envoy to Syria, estimated last month that the actual death toll could be as high as 400,000 people.