Five pro-Iran fighters were killed by unidentified missiles on Wednesday evening in Syria's eastern province of Deir Az Zor, a war monitor said.
"Missiles of unidentified origin targeted the headquarters of the 47th Brigade of pro-Iranian militias in the town of al-Bukamal, in the east of Deir Az Zor province, killing five fighters," the Syrian Observatory for Human Rights (SOHR) said.
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SOHR chief Rami Abdul Rahman said drones may have been responsible for the raids, which caused "loud blasts".
Iran and militias it supports, along with fighters from Iraq and elsewhere, have backed the government of President Bashar al-Assad in Syria's eight-year war.
Israel has promised to prevent its regional rival Iran from gaining a foothold in the country and has carried out hundreds of raids against Iranian targets.
The vast desert province of Deir Az Zor, which neighbours Iraq, hosts several actors in the Syrian conflict besides pro-regime forces.
The Syrian Democratic Forces, a Kurdish-dominated armed group backed by the United States, were instrumental in destroying the so-called "caliphate" declared by the ISIL (ISIS) group.
Five pro-Iranian fighters were killed in similar raids on the outskirts of al-Bukamal on December 8, the SOHR said.
Twenty-eight pro-Iranian fighters, including at least 10 Iraqis, were killed in similar attacks in September.
The Lebanese Shia movement Hezbollah blamed Israel for that attack, quoting a "security source in Syria".
In June 2018, raids in far eastern Syria were also attributed to Israel by a US official, on condition of anonymity. The SOHR said they killed 55 pro-regime forces.
Syria's complex, multi-faceted conflict has left more than half a million people, according to the SOHR, while the UN said at least 400,000 were killed since 2011.
