The official said the strikes were in self-defense after the pro-regime forces began firing artillery and tank rounds at the USA -backed SDF, which is dominated by the PKK terror group's Syrian offshoot the Democratic Union Party (PYD) and its armed wing the People's Protection Units (YPG).
The Pentagon said that earlier on Wednesday evening, an estimated 500 Syrian government fighters and heavy artillery had crossed an agreed "demarcation line" along the Euphrates river and had launched an "unprovoked attack" on the SDF headquarters.
The US-led coalition's attack on pro-government forces in Syria has again proven that Washington's true goal is to capture "economic assets" instead of fighting terrorism, the Russian Defense Ministry said.
This clash, which took place in Khusham near the Euphrates River, is the largest reported to date between USA -backed forces and pro-regime forces in Syria.
"Pro-regime vehicles and personnel, who were turned around and headed back west, were not targeted", she said, noting that the US does not seek a conflict with the Bashar al-Assad regime.
The U.S. official claimed that the pro-Assad forces were "likely seeking to seize oil fields in Khusham that had been a major source of revenue for Daesh from 2014 to 2017". The death toll from Syrian regime air strike on the rebel-held enclave of Eastern Ghouta near Damascus rose to 36, a monitor said.
On May 18, they attacked pro-regime forces that continued approaching coalition troops despite warning shots and Russian attempts to halt the troops.
Nebenzia slammed the US, saying Moscow sees an "intensification" of efforts aimed at "defaming" Russia's efforts to achieve a political settlement.
The second official said given the believed relationship between the contractors and the Russian government, it is hard to imagine that Russian military forces in Syria were not aware of the contractors' activities and involvement in the attack.
Following the incident, the Russian military talked to the coalition through the established communication channels, with the latter stating that Al-Isba was under the control of SDF and U.S. troops.
Khusha is five miles east of the de-confliction line that separates Russian-backed and USA -backed forces in Syria.
The Euphrates valley is one of open fronts still in fight against Islamic State (ISIS).
About 2000 U.S. forces remain on the ground in Syria, allied to the Kurdish-led SDF alliance. The Syrian Observatory said those involved and killed by the strikes were tribesmen loyal to President Bashar al-Assad and Shia militias of Afghanistan.
Veale also confirmed the assumption was that the pro-regime fighters were trying to recapture the oil fields that the USA -backed coalition wrested from ISIS a year ago.
A USA military official who spoke on condition of anonymity said the United States bombing killed about 100 troops, but Veale said the military did not have an exact number for the casualties.
The attack had left "scores of persons dead and others injured" and caused "huge damage" to the area, SANA reported.
Russian Federation on Thursday said the United States goal in Syria was not to fight IS, but to capture the country's economic assets.