The U.S. involvement is a rare example of U.S. forces striking directly at Syrian regime troops. It said it had killed 39 militants.
Neither Washington nor the Western-backed coalition have offered details on the identity of the attacking forces.
According to the Observatory, the forces that launched the attack on SDF positions were local tribal fighters loyal to President Bashar al-Assad and Afghan Shia militia fighting alongside the regime.
The new bloodshed came as Russian President Vladimir Putin and French President Emmanuel Macron discussed cooperating more closely to resolve the Syrian conflict in a phone call Friday.
The Syrian Civil Defense, known as the White Helmets, said the 35 casualties were spread throughout a number of towns in the eastern Ghouta area, which is devastated by a combination of artillery and airstrikes.
In apparent retaliation, one person was killed and five people wounded in rebel mortar fire on the regime-controlled part of the town of Harasta, according to state news agency SANA.
"Children in eastern Ghouta are being starved, bombed and trapped", said Sonia Khush, Save the Children's Syria Response Director.
In a separate statement, UN Secretary-General António Guterres called on the parties involved in the conflict to try and find a political solution.
Recently, a funeral was held at the town of Al-Suqaylabiyah for ten of the ISIS Hunters soldiers killed in the United States airstrikes - many of them were Christians.
But the agency later tweeted that the helicopter was downed in Syria itself, citing the Turkish prime minister, Binali Yildirim.
The overnight attack, which a USA military official earlier said killed about 100 of the attackers, added another layer to the Syrian conflict that is once again plunging into a new and unpredictable spiral of violence.
The report said the action was sanctioned in self-defence after several hundred attackers launched a "coordinated assault" on the Syrian Democratic Forces - a United States backed outfit. Turkey has always been one of the main allies of the anti-Assad rebels.
"I am heartbroken by what the children of Syria continue to suffer because of actions taken by adults - actions that show total disregard for the protection, safety and wellbeing of children", she added.
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