A Syrian opposition group whose members beheaded a child captive near Aleppo on Tuesday had been part of a US-vetted alliance and been supplied with anti-tank missiles until a year ago.



Images of the killing are widely seen as among the most shocking of the war and continued to receive condemnation, with Amnesty International describing the killing as having “truly plumbed the depths of depravity”. A US State Department spokesman described reports of the killing as “appalling” and said proof that the killers were linked to the group, known as Nour al-din al-Zenki, would lead to any further backing being stopped.



Nour al-din al-Zenki had previously been accused of committing atrocities in its stronghold of northern Aleppo. Such claims led to the US cutting ties with the group in early 2015.

As Syria has unravelled over the past four years, abuses have been rampant on both sides, with summary executions, forced disappearances, starvation sieges and indiscriminate targeting of civilians becoming commonplace.



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Amnesty’s Middle East director, Philip Luther said: “All detainees, including captured fighters, must be treated humanely, and children in particular must be protected. This appalling incident appears to point to a pattern of abuses by armed groups in northern Syria who have free rein to commit summary killings, abductions and torture without any prospect of being held accountable.



“In addition to condemning such abuses, leaders of armed opposition groups must remove from their ranks any members suspected of committing serious violations of international humanitarian law. It remains the case that the vast majority of people who have died in custody during the last five years have been in the custody of the Syrian authorities.”



Nour al-din al-Zenki leaders claim to have arrested those involved in the killing and vow they will face a judicial process.



Since 2013, the US had provided military support to a limited number of rebel groups who had allied under the Free Syrian Army, which had been formed in the early months of the war from Syrian military defectors and citizens. Rebel groups had constantly splintered and reconfigured and the ranks of those fighting to oust the Syrian regime were added to by ideologically minded groups, and global jihadis, such as Jabhat al-Nusra and Islamic State (Isis).



Most groups receiving US support were centred around Hama in western Syria and Deraa in the south. Early in 2014, Saudi Arabia began supplying the same groups with Tow missiles – an anti-tank rocket that Washington had sold to Riyadh in large numbers in the early 90s. The end user agreement attached to the sale meant the Saudi supply more than 20 years later had implicit US support.



Throughout the first half of 2015, the Tow missiles were proving to be particularly effective against the ageing armour of the Syrian military. By then, Nour al-din al-Zenki had been cut off from the network, because of regular reports that it had committed abuses. Until then, it had also received supplies of other light weapons from a logistics hub run by the CIA, Saudi Arabia and other allies.

The US arming and training program has since largely collapsed. US officials had remained wary about the loyalties of some of the groups they were backing, while much of the opposition had railed against demands that they fight Isis before turning their weapons on the Syrian regime.



“We hate what they have done,” said a senior member of the Jaish al-Fatah group operating in northern Aleppo. “It is unimaginable to treat children like this. There is no justification for it. And nor is there for what the regime continues to do without the spotlight of scrutiny.”



The US state department said: “We strongly condemn this type of barbaric action, no matter what group is responsible. We do not comment on which groups are funded by the United States. We do, however, routinely vet the groups we work with and support and their human rights record figures prominently in that. We do not support groups that condone this sort of barbarity, period.”