US military says the air strikes were a response to an "unprovoked attack" on the headquarters of the US-backed Syrian Democratic Forces.
The US-led anti-Daesh coalition carried out strikes in "self-defense" against forces loyal to Syrian regime leader Bashar al-Assad, leaving at least 100 dead, the US military said Wednesday.
The strikes were to counter an "unprovoked attack against well-established Syrian Democratic Forces headquarters," the US Central Command said, referring to US-backed forces against Daesh.
"We estimate more than 100 Syrian pro-regime forces were killed while engaging SDF and coalition forces," while one SDF member was wounded, a US military official said.
TRT World's Sara Firth reports from Kilis near the Turkey-Syria border.
The SDF is backed by coalition support on the east bank of the Euphrates. The main component of the SDF, known as the YPG, is the target of a military operation in northern Syria by US ally Turkey, which considers the YPG a "terrorist" group and the Syrian branch of the PKK.
The PKK is regarded as a terror group by the US, EU, NATO and Turkey.
According to CENTCOM, "coalition service members in an advise, assist, and accompany capacity were co-located with SDF partners during the attack."
"The coalition conducted strikes against attacking forces to repel the act of aggression," it said.
The US military official said the attack - which began on Wednesday - involved some 500 personnel from "pro-regime" forces armed with tanks, artillery and mortars.
Statements carried by Syrian state television called it a "new aggression" and "an attempt to support terrorism".
The SDF and the coalition targeted the attacking forces with air and artillery strikes after "20 to 30 artillery and tank rounds landed within 500 meters (yards) of the SDF headquarters location," the official said.
The official did not specifically identify the attacking forces, which could have been Syrian or from one of a number of allied militia units, including from Iraq and Lebanon, that are backing Assad in the country's seven-year civil war.
Russia is carrying out strikes in support of Assad, who has received substantial support from Iran as well.
CENTCOM said the attack occurred eight kilometers (five miles) east of the "Euphrates River de-confliction line," referring to a boundary agreed to by Russia and the US, with the former's area of operations to the west of the river and the latter's to its east.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted" attack, the US military official said, adding that "Russian officials assured coalition officials they would not engage coalition forces in the vicinity."
The attack on the SDF in eastern Syria could spell further trouble for the US-backed forces, which have largely avoided confrontation with pro-Assad units until now.
TRT World's Olly Barratt brings more from Russia.
Russia's UN envoy described a deadly US-led coalition strike on pro-regime forces as "regrettable" and said he would raise it during a United Nations Security Council closed-door briefing on the Syria's humanitarian situation on Thursday.
"That's very regrettable, we will raise that issue, we will ask them what happened," Russian UN Ambassador Vassily Nebenzia told reporters. He said he did not believe any Russians had been in the area where the strike happened.
The Syrian conflict began in 2011 after a crackdown on anti-regime and pro-democracy protests.
The war has claimed more than 400,000 lives and triggered one of the world's worst humanitarian crises in recent history.