USA personnel have been advising and assisting the SDF, and some were embedded with them at the time of the attack in Deir el-Zour province.
"International coalition forces are bombing popular forces fighting "Daesh" and the SDF [Syrian Democratic Forces] east of the Euphrates river and there are reports of casualties", the statements said.
"We estimate more than 100 Syrian pro-regime forces were killed while engaging SDF and coalition forces", an official said, adding that the strikes targeted almost 500 "pro-regime" forces armed with tanks and artillery.
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 USA troops.
According to the USA -led alliance, there were some coalition troops embedded with the Kurdish-Arab SDF alliance when the attack took place. Officials declined to say whether the airstrikes were conducted with manned or unmanned aircraft.
Some 100 pro-regime forces were killed in the incident, according to the broadcaster.
The comments by Mattis came on the heel of deadly US-led coalition air strikes which killed some 100 pro-Syrian government forces.
In the eastern Syrian province of Deir az Zur UDF struggles against the Islamic State group east of the Euphrates.
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Before Turkey's Afrin operation in January Kurdish sources said they had been pressured to hand over the gas field in exchange for the regime preventing a Turkish offensive against the People's Protection Units (YPG) in Afrin.
Damascus has also urged the United Nations to press the USA to leave, especially following the successful defeat of the main IS forces.
The Syrian Observatory said they were local tribesmen loyal to President Bashar al-Assad and Afghan Shia militiamen.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted (pro-regime forces) attack", Veale said.
The US version of events, however, raises "lots of questions", Russian Foreign Ministry spokeswoman Maria Zakharova said Thursday. That official also said the military assesses that some of the Russian contractors were killed in the USA counterstrike.
Those pro-regime vehicles and personnel who retreated were not targeted by coalition air and artillery strikes, he said. "Russian officials assured Coalition officials they would not engage Coalition forces in the vicinity".
In a statement, it said 25 militia were wounded, and accused the USA of being motivated by economic concerns as the strike took place near an oil refinery.
"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 Global Coalition's defeat-Daesh mission", a CENTCOM statement said, referring to the alternative label for Islamic State militants. The Syrian army is supported by Iranian-backed militias and Russian forces.
The US-led coalition backs a Kurdish-dominated ground force in eastern Syria, which has an increasingly uneasy and sometimes antagonistic relationship with Mr Assad, who is also backed by Iran. A pro-regime drone was also grounded after it dropped a munition in the vicinity of USA troops in Syria. A total of 25 pro-government fighters were wounded.