The Syrian Observatory for Human Rights said "23 pro-Iranian fighters", including five Syrians, four from Lebanon's Hezbollah group, six Iraqis and eight Iranians, were killed in at least nine pre-dawn air strikes near the Iraqi border.
It said the raids were "likely carried out by Israel", after earlier indicating they were "likely American".
A US military official, requesting anonymity, said the "US did not conduct any defensive strikes overnight".
The Observatory said the strikes targeted military positions in Albu Kamal and its surroundings in Deir Ezzor province, adding that a weapons shipment from Iraq and an ammunition warehouse were also hit.
Israel rarely comments on individual strikes targeting Syria, but it has repeatedly said it will not allow arch foe Iran, which backs President Bashar al-Assad's government, to expand its presence there.
Also Saturday, the Observatory said that "Israeli missiles targeted warehouses and bases of pro-Iran groups" near airport in the main northern city of Aleppo, killing four foreign fighters.
Syrian state media, citing a military source, said that at around 5:20 pm (1420 GMT), "the Israeli enemy carried out an air attack... targeting a number of points south of the city of Aleppo".
During more than a decade of civil war in Syria, Israel has launched hundreds of air strikes on its territory, primarily targeting Iran-backed forces including Lebanese Hezbollah fighters as well as Syrian army positions.
But it has intensified attacks since its war with Hamas began on October 7.
Hezbollah announced on Saturday the death of four of its fighters, without saying when or where they were killed.
There have been regular cross-border exchanges of fire between Israel and Hezbollah, a Hamas ally, in southern Lebanon.
Early on Saturday in southern Syria's Quneitra province, near the Israeli-occupied Golan Heights, "Israeli ground bombardment" killed two fighters from a Hezbollah-linked group, the Britain-based Observatory said.
The Israeli army said that it carried out multiple strikes in Syria after two rockets fired from the country hit areas under its control.
The Middle East has also seen a surge in attacks on US forces, which Washington blames on pro-Tehran groups.
The majority have been claimed by the Islamic Resistance in Iraq, a loose formation of Iran-linked armed groups which oppose US support for Israel.
The Observatory reported attacks on two US bases in Deir Ezzor province late Saturday. It said the rocket and drone assault was carried out by Iran-backed groups.
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