Violent explosions were heard from missile stockpiles of pro-Iran militias east of Syria's capital Damascus before dawn on Sunday, a war monitor said.
Residents of the Damascus region heard the blasts which came from "the warehouses of pro-Iran militias" in a mountainous area east of the capital, said the Britain-based Syrian Observatory for Human Rights, which relies on a wide network of sources inside Syria.
"We don't know if it was from an air strike or ground operation," Observatory head Rami Abdel Rahman told AFP.
During more than a decade of war in Syria, neighbouring Israel has launched hundreds of air strikes on Syrian territory, primarily targeting Iran-backed forces and Hezbollah fighters, as well as Syrian army positions.
Syria's official news agency SANA said during the night that "the sounds of explosions" had been heard on the outskirts of Damascus.
Four Syrian soldiers and two Iran-backed fighters were killed last Monday in pre-dawn Israeli air strikes near Damascus, the Observatory said at the time, in the latest deadly Israeli air raid to hit war-torn Syria's capital.
The air strikes targeted Syrian regime forces, as well as military positions and weapons depots used by armed groups supported by Tehran, the monitor said.
With Iranian as well as Russian support, the government of Syria's President Bashar al-Assad has clawed back much of the territory it had lost to rebels early in the conflict, which broke out in 2011 and has pulled in foreign powers and global jihadists.
Israel rarely comments on strikes it carries out on targets in Syria, but it has repeatedly said it would not allow its arch foe Iran to expand its footprint there.
Syria's war has killed more than half a million people and displaced millions.