However, during the mission, the Syrian militiamen were subjected to sudden shelling from mortars and multiple launch rocket systems and then came under fire from helicopters of the US -led coalition, the statement said.
U.S. Sen. Tim Kaine, a member of the Senate Foreign Relations and Armed Services Committees, criticized the U.S. strikes.
"Let's be realistic", he said.
"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 fight against Islamic State (IS) extremists, the statement said.
The pro-government forces were "likely seeking to seize oilfields in Khusham" east of the Euphrates in Deir al-Zor province, a United States official said. At least 25 of the popular forces were injured in the attack, the Russian MoD noted.
Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel.
This is not the first time the USA military has clashed with pro-regime forces in Syria.
During the attack, one SDF soldier was wounded, and there were no coalition casualties, according to White.
The Russian Interfax news agency quoted Franz Klintsevich, a Russian parliamentarian, condemning the airstrikes.
"It's noteworthy that the extremists themselves and weapons for them, according to numerous witness accounts, are often transferred to the territory of Afghanistan by helicopters without identifying insignia", he was quoted as saying.
"We will continue to support them".
While Washington's mission is to finish off IS and ensure it does not return, the US also has a stake in stabilizing swaths of Syria.
US F-22 stealth jets, F-15 fighters, and MQ-9 drones were involved in the counterattack according to US Air Forces Central Command.
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What does Syrian media say?
The drastic escalation in fighting has tightened the siege on some 400,000 people who are trapped in Eastern Ghouta with little access to food and medicine.
Mattis said the Russians had told American military officials that they did not have any Russian forces there.
The coalition said the attack took place in Khusham in the Middle Euphrates River Valley, five miles east of the agreed-on "de-confliction" line created to separate Russian-backed regime troops and US-backed forces in Syria.
Washington says it believed it was targeting an IS position there, but Damascus says the strike was "intentional". Just this week, the two countries clashed at the United Nations over Syria's use of chemical weapons.
Ikhbariya described the groups it said had been bombed by the USA -led coalition as "local people fighting [ISIS] and the SDF", Reuters reported.
Separately, the USA -led air and artillery strikes began overnight Wednesday in Deir al-Zour province.
About 2000 U.S. forces remain on the ground in Syria, allied to the Kurdish-led SDF alliance.
Russian, Turkish, and Iranian leaders reportedly planned to meet to discuss Syrian peace efforts.
The officials said that Turkish President Recep Tayyip Erdogan and his Russian counterpart Vladimir Putin agreed to the summit during a telephone call on Thursday.
It was not immediately clear when the Istanbul meeting would take place.
Currently, the YPG is also fighting off a Turkey-led invasion in northern Syria's Afrin region. The official spoke to Reuters on condition of anonymity. teleSUR Middle East correspondent Hisham Wannous confirmed the reports.
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