Military aircraft have struck positions of Iran-backed militias near Syria's border with Iraq. at least 18 Iranian and pro-Iranian fighters were killed.
It was not clear who made the overnight strikes in and around the city of Albu Kamal.
But Israel has carried out hundreds of attacks against Iran-related targets in Syria during the country's civil war.
It seeks to thwart what it calls the "military introduction" of Iran into Syria and the supply of Iranian weapons to battle groups such as Hezbollah in Lebanon.
The Syrian Observatory for Human Rights cites its sources, that unidentified military aircraft bombed bases, weapons depots and les vehicles belonging to Iran-backed militias in al-Hizam al-Ahdar and other areas around Albu Kamal.
Omar Abu Laila, a European activist from the DeirEzzor 24 group, said powerful explosions were heard in the city and that
Another Sound and Picture activist cited medical sources that 21
A source from Iran-backed Iraqi paramilitary forces, a popular mobilization, told the BBC that a drone targeted its headquarters on the Iraqi-Syrian border, about 6 km (4 miles) south of Albu.
The force sent vehicles from the nearby Iraqi city of al-Qa'im because of the high number of casualties, the source added.
Hours later, a number of rockets were fired at Israel's outskirts of the Syrian capital, Damascus, but we were unable to strike the territory of Israel, according to the Israeli military.
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Blames militia commanders under the command of the Iranian Revolutionary Guard's Quds Force, but said it held the Syrian government accountable,
Meanwhile, Hezbollah said its fighter jets dropped an Israeli drone drone over southern Liz on Monday, and they seized it.
The Israeli army stated that one of its drones "fell in southern Lebanon during routine operations". This did not justify, but said there "no information of concern can be obtained".