Israel has ramped up strikes on Syria recently, including in areas near the Lebanese border mainly targeting bastions of Iran-backed Hezbollah. Israel has been at war with the Lebanese group since September.

"The death toll from the Israeli strikes on the Mazzeh neighbourhood and Qudsaya rose to 20 people, in addition to 21 other wounded," the Britain-based Syrian Observatory for Human Rights said.

The Mazzeh neighbourhood, home to embassies, United Nations offices and security headquarters, has been the target of previous strikes blamed on Israel.

Qudsaya is located on the outskirts of Damascus.

"Israeli strikes destroyed three multi-storey buildings in the Mazzeh neighbourhood, killing 10 people," said the Observatory with a network of sources inside Syria. It added that the dead included at least three civilians and two non-Syrian Iran-backed fighters.

In Qudsaya, Israeli jets targeted "an apartment complex housing Palestinians, killing 10 people, including at least three members of the Palestinian Islamic Jihad movement," the monitor said.

Islamic Jihad has fought alongside Hamas against Israel in Gaza.

Earlier, Syria's defence ministry said the twin Israeli air strikes killed 15 people after "targeting residential buildings in the Mazzeh neighbourhood of Damascus and the Qudsaya area in the Damascus countryside".

The official SANA news agency published video footage of smoke covering a street.

Early last month Syria's government said seven civilians were killed in an Israeli air strike, in the Mazzeh district, which the Observatory said targeted a building used by Iran's Revolutionary Guards and Hezbollah.

In April, Syrian and Iranian officials blamed Israeli air strikes for the destruction of Iran's embassy consular annex in Mazzeh. The strike killed seven members of Iran's Revolutionary Guards.

That attack led to Iran's first ever direct strike against Israeli soil, a barrage of drones and missiles, which in turn led to an apparent Israeli retaliation, raising fears of regional conflagration.

Iran and Lebanon's Hezbollah have been among the Syrian government's most important allies in the country's civil war that began in 2011.

Israeli authorities rarely comment on individual strikes in Syria but have repeatedly said they will not allow arch-enemy Iran to expand its presence.