An air strike in eastern on Sunday killed five non-Syrian backed by , a UK-based war monitor said.
The strike targeted three military vehicles belonging to Iran-backed paramilitary fighters near the Iraqi border, the Syrian Observatory for Human Rights said.
The monitor did not say who was behind the attack near the border town of Albu Kamal.
But Observatory head Rami Abdul Rahman said that " was likely responsible".
Israel has hundreds of strikes in Syria since the start of the civil war in 2011, targeting government troops, allied Iranian forces and fighters from the Lebanese militant group Hezbollah.
It rarely confirms details of its operations in Syria, but says Iran's presence in support of President Bashar al-Assad is a threat and has vowed to continue its strikes.
Iranian-backed militias and their allies command a significant presence in eastern Syria south of the Euphrates Valley.
Earlier this month, air strikes on a base controlled by Iran-backed forces near Albu Kamal killed seven fighters, according to the Observatory.
The monitor said that military reinforcements for Iranian militias and their allies had arrived in the area days before.
Israel did not immediately comment on the attack.
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