More than 100 fighters allied with Syrian President Bashar al Assad have been killed in US-led coalition air and artillery strikes on pro-regime forces.
The air assault was launched after up to 500 pro-regime fighters backed by artillery, tanks, multiple-launch rocket systems and mortars targeted the Syrian Democratic Forces (SDF) headquarters in Deir al-Zor province in an “unprovoked attack”.
Between 20 to 30 artillery and tank rounds landed within 500m of the HQ, where coalition advisers are working with SDF fighters battling Islamic State militants east of the Euphrates River in Syria.
The Syrian army, which is supported by Iranian-backed militias and Russian forces, are active on the other side of the river by the city of Deir al-Zor.
The US-led coalition had previously alerted Russian officials about the presence of SDF forces in nearby Khusham after seeing a slow build-up of pro-government forces during the last week.
Details of the thwarted attack were released by US officials who wished to remain anonymous.
Coalition officials were in regular contact with their Russian counterparts before, during and after the attack, who had assured them that they would not target US and SDF forces in the area.
No American troops were wounded or killed in the attack, however one SDF member was hurt.
Officials believe the attack was an attempt by pro-regime forces to take back land the SDF had liberated from IS fighters in September, including oil fields in Khusham.
Interfax news agency quoted Franz Klintsevich, a Russian parliamentarian, condemning the airstrikes: “The actions of the US coalition do not comply with legal norms, beyond all doubt it is aggression.”
Meanwhile, the UN humanitarian coordinator for Syria, Panos Moumtzis, has appealed for an immediate ceasefire as the Syrian conflict escalates.
“We have multiple fronts with people in extreme danger without a view to a solution,” he said.
Members of the UN Security Council are meeting later behind closed doors to discuss the situation.
It comes after 19 children were killed in a round of airstrikes by the Syrian regime on a rebel enclave near the capital Damascus.
The town of Douma in eastern Ghouta was under siege on Wednesday, with the Syrian Observatory for Human Rights reporting that 34 people had been killed and about 200 more injured in the bombardment.
It came less than 24 hours after the region – controlled by rebel factions, including Islamists – suffered some of its worst bloodshed in years, with 80 civilians left dead by regime warplanes.
More than 340,000 people have been killed and millions displaced since the conflict in Syria erupted in March 2011.