On Saturday, air strikes blamed on Israel hit positions belonging to regime forces and Iran-backed militias near the border with Iraq, a war monitor said.
Air strikes targeting positions of Iran-backed militias in eastern Syria killed nine fighters in the second such raid in 24 hours, a war monitor said on Sunday.
The Britain-based Syrian Observatory for Human Rights said Israel was "likely responsible" for the strikes near the Iraqi border.
They came hours after a similar raid killed six other Tehran-backed fighters, raising the total toll to 15 killed in 24 hours, according to the monitor.
The fighters killed in the early Sunday raids were mostly Iraqi nationals, according to Observatory head Rami Abdul Rahman.
There was no official comment from Israel.
Israel has launched hundreds of strikes in Syria since the start of the civil war in 2011.
It has targeted Syrian regime forces, allied Iranian forces and fighters from the Lebanese Shia militant group Hezbollah.
It rarely confirms details of its operations in Syria but says Iran's presence in support of regime leader Bashar al Assad is a threat and that it will continue its strikes.
Concerns over Israeli agents?
On Saturday, air strikes also blamed on Israel hit positions belonging to regime forces and Iran-backed militias near the border with Iraq, the Observatory said.
Four Syrian nationals were among the six fighters killed in that attack, the monitor added.
Saturday's raids came only days after Israeli strikes in the eastern province of Deir Ezzor and the southern province of Suweida killed seven fighters, including two Syrian regime fighters, according to the Observatory.
In early June, Israeli strikes killed at least nine fighters loyal to the Syrian regime, including four Syrians, in Hama province – a central region controlled by the Syrian regime forces and Iranians, according to the Observatory.
The uptick in attacks has prompted concern among Iran-backed forces in east Syria that Israeli agents may be among their ranks, the monitor said.
These forces have arrested four people on suspicion of providing intelligence to Israel, the war monitor reported on Sunday, shortly before the latest raids.
The war in Syria has killed more than 380,000 people and displaced more than half of the country's pre-war population since 2011.