US-led warplanes carried out Thursday'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.
The Pentagon says this was a defensive strike, launched after pro-regime forces had violated an agreed deconfliction zone around a base at al-Tanf, near the Syria's southern border with Jordan and Iraq, where US and British Special Forces are training rebels to fight ISIS.
The airstrikes marked the first time that US military directly targeted troops loyal to Syrian dictator Bashar Assad since President Trump authorized the launch of 59 Tomahawk missiles at a Syrian base in early April.
"The new USA strike on government troops in Syria is a continuation of Washington's non-constructive policy", said Leonid Slutsky, the head of the International Affairs Committee of the State Duma, the lower chamber of the Russian parliament. "Of course, it does not help the political process", Russian Deputy Foreign Minister Gennady Gatilov stated on Friday.
US-led anti-ISIS coalition has carried out air strikes against a convoy of a Iran-backed Shiite militia supporting the regime of Syrian President Assad.
The source said the attack hampered efforts by the Syrian army and its allies to fight Daesh terrorists who have been wreaking havoc in Iraq and Syria for years.
The U.S. military does not speak with the Syrian government.
While officials stressed the defensive strike did not signal deepening U.S. involvement in Syria's civil war, it nonetheless raises questions about the ongoing feasibility for the coalition to maintain its singular focus on the so-called Islamic State group. But the Russians and pro-regime forces apparently ignored warnings, and the United States then hit the convoy with a direct airstrike.
In a second incident near al-Tanf, U.S. fighter jets intercepted a Syrian SU-22 fighter-bomber that had entered air space near the base.
"We notified the coalition that we were being attacked by the Syrian army and Iranians in this point, and the coalition came and destroyed the advancing convoy", he said.
The United States is not deepening its role in Syria's civil war but will act defensively when needed, US Defence Secretary Jim Mattis said on Thursday.
The Russians relayed the message to the Syrian government, but the forces did not withdraw, the official said. "Trump does not have Congressional authorization to attack Syria, a country that has not attacked USA".