Around 100 soldiers allied with Syrian President Bashar al-Assad have been killed in airstrikes and artillery strikes by a US-led coalition.
The rare strikes against pro-regime forces were confirmed by the US-led coalition Operation Inherent Resolve, which is in the region targeting Islamic State.
Coalition forces said they airstrikes were in response to an 'unprovoked attack' by Syrian pro-regime forces against a Syrian Democratic Forces (SDF) headquarters in Deir al-Zor province in eastern Syria, near the border with Iraq.
Hundreds of pro-regime fighters are reported to have taken part in the attack, which is believed to have left one SDF member injured.
In a statement, Operation Inherent Resolve officials said: "Coalition service members in an advise, assist, and accompany capacity were co-located with SDF partners during the attack eight kilometers east of the agreed-upon Euphrates River de-confliction line.
"In defense of Coalition and partner forces, the Coalition conducted strikes against attacking forces to repel the act of aggression against partners engaged in the Global Coalition's defeat-Daesh mission."
The Russian Interfax news agency quoted Franz Klintsevich, a Russian parliamentarian, condemning the airstrikes.
He said: "The actions of the US coalition do not comply with legal norms, beyond all doubt it is aggression."
The conflict in Syria has seen more than 300,000 people killed and millions more displaced since it began in early 2011.
It comes as the United Nations has issued a fresh warning about the 'dire humanitarian crisis' in parts of Syria, saying there are people across the country in 'extreme danger'.
Officials with the UN humanitarian team in the country said: "In this extreme situation, an immediate cessation of hostilities lasting for at least one month throughout Syria to enable the delivery of humanitarian aid and services, evacuation of the critically sick and wounded, and alleviation of people’s suffering, to the extent possible, wherever they are."
Recent days have seen dozens of people killed in the rebel-held areas of Eastern Ghouta and Idlib province, which have been targeted by Russia-backed Syrian government forces.
Al Jazeera reports that the death toll in the region had climbed to at least 180 people following four days of air attacks.