Russian Deputy Foreign Minister Gennady Gatilov hit out at the attack Friday, according to state news agency RIA-Novosti.
"Any military actions leading to the aggravation of the situation in Syria definitely affect the political process, especially if such actions are committed against the Syrian armed forces," Gatilov is quoted as saying. "This is totally unacceptable; it is a violation of Syrian sovereignty."
The US said Thursday it had struck a convoy of 20 pro-regime vehicles in Syria a day earlier.
It is the second time since Donald Trump became President that the US has intentionally struck pro-regime forces in Syria.
Direct threat to 'US partner forces'
According to a US defense official, a convoy of 20 pro-regime vehicles was headed toward al-Tanf, a military base on the Syrian-Jordanian border, on Wednesday night.
Al-Tanf, which Russian warplanes bombed a year ago, is currently occupied by US and British Special Forces that are advising an anti-ISIS Syrian rebel group known as Maghawir al-Thawra, or the Commandos of the Revolution.
Thirteen of the vehicles apparently breached the "de-confliction zone" around the base, an area that the coalition has communicated to Moscow to stay well clear of.
US warplanes were first scrambled in a "show of force" against the oncoming convoy. But then five vehicles kept approaching, coming within 29 kilometers of the base when they were finally hit by US aircraft.
The coalition confirmed
that the convoy posed a direct threat to "US partner forces" -- "despite Russian attempts to dissuade pro-regime movement" toward the base.
A message for Iran?
The strike marks the first time that the Pentagon has offered aerial protection to its Arab proxies under assault from pro-Syrian militias -- and only the second time in the history of the six-year conflict that American warplanes have intentionally targeted Iranian proxies in Syria.
The convoy appears to have consisted not of regular Syrian army soldiers but of international Shia militiamen.
The news of the strikes came just hours before Trump was set to make his first overseas trip as President.
Trump will travel to Saudi Arabia on Friday
, where he is expected to reassure America's Sunni Gulf allies that his administration is committed to containing and deterring Iran, which is now the principal security underwriter of the Assad regime in Syria.
Iran's expansionist ambitions have begun to chafe under a slowly increasing US hard power deployment in the region, opposition sources have told CNN.
Tlass Salameh, the commander of the Lions of the East Brigade, said that his men are located 20 kilometers from al-Tanf. The brigade receives support from a covert CIA program designed to train and arm vetted Syrian rebel groups, according to Salameh.
Recruits of that program are allowed to fight the Syrian regime and its allied militias, including those imported from Lebanon and Iraq and beholden to Iran's Revolutionary Guards Corps' Quds Force.
On Monday, Iran's state-run Fars News Agency claimed
that Lebanese Hezbollah, a prominent Iranian proxy, had deployed "12 regiments with 1,000 fighters" to southern Syria "to face the US-backed militants in al-Tanf border crossing."
"The regime is bombing us in 25 to 30 raids on a daily basis. Russia hit us once or twice," Salameh told CNN. "We have a post in Mafraq Kabid on the Damascus-Baghdad highway, which is now controlled by the Iranian and [Lebanese] Hezbollah militias."
That transnational highway is crucial to Iranian plans to construct a ground corridor, or land bridge, from Tehran all the way to Mediterranean coast.
Rebel sources confirm a report published this week by the UK's Guardian newspaper
to that effect, noting that the original route for this corridor has recently shifted from northern Syria, running through the heartland of Syrian Kurdish territory, to Sunni Arab tribal south of the country.
The Guardian suggested that the change of latitude owed to a growing presence of US troops and US-run military installations the north, used by various anti-ISIS forces.
Syria condemns 'aggression'
A Syrian military source also reacted with anger to the US coalition strike, Syrian state media reported Friday.
"The attempt to justify this act of aggression by saying that the targeted forces failed to respond to the warning to stop advancing is completely rejected," the source told SANA, the Syrian Arab News Agency.
"The Syrian Arab Army," the source said, "is fighting terrorism on its territory, and no party whatsoever has the right to determine the course and destination of the army's operations against the terrorist organizations, mainly ISIS and al-Qaeda."