The footage circulated on social media on Tuesday showed a member of the Nureddin al-Zenki opposition group cutting off the small boy's head with a knife on a public road in Aleppo.
Nureddin al-Zenki, which is largely based in Syria’s Aleppo Province, has received military support in the past from the United States as part of Washington's backing for "moderate" militants in Syria.
Kerry said Tuesday he had seen the “appalling report" about the group's beheading of the boy.
"Obviously we’re very concerned certainly if it’s accurate,” Kerry said, adding if the report is proven it would “give us pause about any assistance or, frankly, any further involvement with this group.”
The boy was identified by activists on social media as Mahmoud Issa, a 12-year-old Palestinian.
Before being killed, the boy is seen on the back of a truck surrounded by militants who accuse him of being a spy and a member of al-Quds Brigades, the armed wing of the Palestinian Islamic Jihad resistance movement.
However, the Palestinian group issued a statement, strongly denying that Issa was a member, adding that the boy was ill. It said he was killed by a “terrorist” who lost his brother in battles near Handarat apparently as revenge.
This screen grab shows a so-called Free Syrian Army militant operating a US-made BGM-71 TOW missile in Syria.
The Syrian government also condemned the “repulsive crime” against an innocent child in a statement to the UN, saying the boy was a Palestinian from the refugee camp of Handarat on the edge of Aleppo.
Amnesty International said the video was the latest “abhorrent signal” that the militant groups are carrying out serious abuses with impunity.
“This horrific video showing the beheading of a boy suggests some members of armed groups have truly plumbed the depths of depravity," Philip Luther, director of Amnesty International’s Middle East and North Africa Program, said.
"It is yet another gruesome example of the summary killing of captives, which amounts to a war crime,” he added.
Lebanese resistance movement Hezbollah also issued a statement, denouncing the beheading as a “disgusting crime” by “so-called moderate” militants “backed by the US and Saudi Arabia.”
The Syrian army is currently fighting foreign-backed terrorists in Aleppo. On Sunday, government troops completely cut off the only main road used by foreign-backed militants into Syria's second biggest city.
Kerry said he had urged Russia to use its influence on Syria to halt military attacks on what it regards as moderate groups and “civilians.” Those remarks came after airstrikes by the US and France inside Syria killed as many as 140 civilians in Aleppo.
Kerry emphasized the need to end all attempts to besiege the city of Aleppo and other besieged towns, according to State Department Spokesperson John Kirby.
File photo shows Takfiri militants in Syria
Last month, Russian Foreign Minister Sergei Lavrov said the US had asked Moscow to stop conducting airstrikes against Nusra Front terrorists in Syria.
Russia has long insisted that the “moderate” militants supported by the US in Syria should leave the areas held by Nusra militants, which are targeted in the Russian air campaign.
Claiming it is unable to remove the opposition groups, Washington, however, has called on Moscow not to carry out airstrikes against Nusra terrorist group.
In May, Britain, France, the US and Ukraine blocked Russia’s request to add Jaysh al-Islam and Ahrar al-Sham militant groups to a UN terror blacklist and sideline them from the Syrian peace process.
Meanwhile, many members of "moderate" groups have joined the most brutal militants with their advanced weapons, including US-made BGM-71 TOW missiles which terrorists widely use in their attacks against the Syrian government.
Takfiri 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, Press TV reported.
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