Russian and American warplanes confronted one another in the skies above Syria after Moscow bombed an elite force of US-trained rebels.
The Pentagon on Friday scrambled F/A-18 fighters to fend off an attack by Moscow’s most advanced bombers on moderate opposition fighters it is supporting.
When the F/A-18s approached the Russians moved out of the area, but when the US fighters paused to refuel they returned and struck the base again, in what appeared to be a deliberately provocative move.
The Russians had been ordered to back off by the US pilots directly using a special communications channel set up to prevent air accidents, but were ignored.
Shortly after, US military officials held a video conference with Russia in which they “expressed strong concerns about the attack on the coalition-supported counter-Isil forces at the Al-Tanf garrison, which included New Syrian Army forces that are participants in the cessation of hostilities in Syria,” said Pentagon spokesman Peter Cook.
The Russians claimed on Sunday that the location given by the US did not match what was on the ground.
“The Russian side explained that the target that was bombed is 300 kilometres from the area identified by the Americans as the location of Syrian opposition groups participating in the ceasefire,” said Major General Igor Konashenkov, a Russian military spokesman, commenting on talks with the US after the incident.
The New Syrian Army (NSA), which receives training and direction from British, American and Jordanian special forces, said their base had been struck with cluster bombs.
The strike left two people dead and another 18 injured, appearing to incapacitate at least half of the force and drawing a furious reaction from Washington.
Although Russia has justified military intervention in Syria with the need to fight extremist groups, its air strikes have strengthened President Bashar al-Assad’s hand and sapped rebel groups that had been fighting Isil.
The Pentagon’s $500m programme to train Syrian rebels in the fight against Isil has faced scathing criticism for being ill-conceived and largely ineffectual.
But experts say the NSA stands out as a rare success story – and it was slated to lead a crucial assault to split the extremist group’s so-called caliphate straddling Syria and Iraq.
The Kremlin appeared to confirm involvement in the attack on Friday when a spokesman, Dmitry Peskov, told reporters it was difficult to distinguish between mainstream and extremist rebel groups from the air.
Monitors say there is no known jihadist presence around al-Tanf.
The direct assault on one of Washington’s most important anti-Isil proxies will further shake the frail relationship of convenience that has developed between the US and Russia as both sides try to bring an end to Syria’s bloody civil war.
Tensions ratcheted up on Friday after news broke of an an internal document in which dozens of US State Department employees called for military action against Mr Assad’s forces.
Mr Peskov warned Washington that strikes against the Syrian army could “plunge the region into total chaos.”
Khaled al-Hammad, one of the grop’s founders, said on Friday that the al-Tanf base had only housed around 30 fighters before the attack, which killed two and wounded 18. Asking why the US had not come to the unit’s aid in their hour of need.
“We do not think they are serious about us and that is why the Russians bomb us. Because they know the US will not come to our defence, like they do with the YPG and SDF,” he said, referring to a the Kurdish-dominated force that Washington has preferred to lead the anti-Isil fight in northern Syria.
The NSA is largely made up of defected Syrian special forces and has been equipped with state-of-the-art US weaponry. The Pentagon has also based M142 high-mobility artillery rocket systems in Jordan to protect the group’s al-Tanf based.
It is understood that Washington planned to back the group in the battle to retake al-Bukamal, a strategically vital border town connecting Isil’s territory in Iraq and Syria. The unit has the potential to win support there because its members are largely drawn from an umbrella fighting group in the nearby province of Deir Ezzor, now an Isil stronghold.
“Militarily this is critical to beating Isil in Syria,” said Faysal al-Itani, an expert on south Syrian rebel groups and a fellow with the Atlantic Council.
But the NSA was already embattled, even before the Russian attack and its fighters complain they are not receiving the support they were promised from the West. In April, 11 of their number were killed by an Isil car bomb.
Pleas for US back-up were not answered swiftly enough to end a follow-up attack. British troops later crossed into Syria from Jordan to help rebuild the group’s shattered defences.
By Louisa Loveluck & Josie Ensor