Syria today condemned a US-led coalition strike on pro-government forces as a "brazen attack" and said it would "not be intimidated" after the surprise assault.
Russia condemned the strike as "illegitimate" and said it could harm efforts to fight Islamic State (IS) group jihadists.
US-led warplanes carried out yesterday's strike in the east of the country against a convoy of pro-government forces headed towards a remote coalition garrison near the border with Jordan.
"On Thursday at 16:30, the so-called international coalition attacked one of the Syrian Arab Army's positions on the Al-Tanf road in the Syrian Badia region, producing a number of martyrs and causing material damage," a military source told Syrian state media.
"This brazen attack by the so-called international coalition exposes the falseness of its claims to be fighting terrorism," it added.
"The Syrian Arab Army is fighting terrorism on its territory, and no party has the right to determine the course of its operations," the source said.
"The Syrian Arab Army will...Not be intimidated by the attempts of the so-called coalition to stop it from performing its sacred duties."
Syrian ally Russia also denounced the US-led strike.
"It is illegitimate, unlawful and yet another gross violation of Syria's sovereignty," Russian Foreign Minister Sergei Lavrov said in televised comments while on a trip to Cyprus.
In a statement, the US-led coalition said it had struck "pro-regime forces...That posed a threat to US and partner forces."
The coalition said the strike came after unsuccessful "Russian attempts to dissuade Syrian pro-regime movement" as well as "a coalition aircraft show of force, and the firing of warning shots".
Lavrov said he was "unaware" that Russia had been given any warning of the strike, and said preliminary information suggested there were civilian casualties.
"We are still confirming the details but according to some reports several dozen civilians died as a result of this strike," he was reported as saying by Russian media.
Syrian state media gave no precise toll in the attack, but the Syrian Observatory for Human Rights, a Britain-based monitor, reported eight killed, "most of them non-Syrian".
A statement from Centcom, the US military command in the Middle East, said two strikes "destroyed two front-end loaders, a piece of construction equipment, a tactical vehicle and a tank".
An array of regular and irregular forces are battling alongside the government against rebels, including Russian and Iranian soldiers and militants from Iraq and Lebanon's Hezbollah group.
Yesterday's strike comes in the context of growing tension over which forces will take on IS in Syria's east.
President Bashar al-Assad's army is trying to prevent US-backed forces from leading that fight.