Five pro-Iranian fighters were killed by unidentified missiles on Wednesday evening in Syria's eastern province of Deir Ezzor, a war monitor said.
"Missiles of unidentified origin targeted the headquarters of the 47th Brigade of pro-Iranian militias in the town of Albu Kamal, in the east of Deir Ezzor province, killing five fighters," the Syrian Observatory for Human Rights said.
Observatory chief Rami Abdul Rahman said drones may have been responsible for the strikes, which caused "loud blasts".
Iran and militias it supports, along with fighters from Iraq and elsewhere, have backed the regime of President Bashar al-Assad in Syria's eight-year war.
Israel has vowed to prevent its regional arch-rival Iran from gaining a foothold in the country and has carried out hundreds of strikes against Iranian targets.
The vast desert province of Deir Ezzor, 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, was instrumental in destroying the self-declared "caliphate" of the Islamic State group.
Five pro-Iranian fighters were killed in similar raids on the outskirts of Albu Kamal on December 8, the Observatory said.
In September, 28 pro-Iranian fighters including at least 10 Iraqis were killed in similar strikes.
The Lebanese Shiite movement Hezbollah blamed Israel for that attack, quoting a "security source in Syria".
In June 2018, strikes in far eastern Syria were also attributed to Israel by an American official, on condition of anonymity. The Observatory said they killed 55 pro-regime forces.
Syria's complex, multi-faceted conflict has left more than 370,000 people dead and displaced millions since it began in 2011 with anti-government demonstrations that were brutally repressed.