Israeli warplanes have fired missiles at targets near Syria's capital, Damascus, Syrian state media report.
The Sana news agency said air defences intercepted most of the missiles, but that eight people were wounded.
A monitoring group said Syrian army positions and those of Iran-backed militias were hit, killing 23 people.
Israel did not comment, but it has acknowledged carrying out hundreds of strikes in Syria in recent years to stop Iranian "military entrenchment".
Iran and its proxies, including the Lebanon's Hezbollah movement, are supporting forces loyal to President Bashar al-Assad in the country's civil war.
Sana cited a Syrian military source as saying that Israeli warplanes flying over Lebanon and the occupied Golan Heights launched two waves of missiles at southern Syria early on Thursday.
The first wave targeted military positions in the suburbs of Damascus, while the second targeted those in Deraa, Quneitra and Damascus Countryside provinces, according to the source.
"[The] vigilance of our air defence personnel contributed to destroying large numbers of the hostile missiles and the aggression caused the injury of eight fighters, in addition to a material damage," the source added.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said strikes on Iran-backed militia positions in Kiswa, Muqaylabiya, Jisr al-Baghdad and Izra killed 15 fighters, including five Syrians and at least three Iranians.
And eight Syrian soldiers were killed by missiles that hit air defence batteries near Damascus, it added.
The Syrian military accused Israel of launching an "aggressive escalation" in an attempt to "save the armed terrorist organisations which have been collapsing in Idlib and western Aleppo in front of the strikes of the Syrian Arab Army".
In recent weeks Syrian soldiers, supported by Iran-backed militiamen and Russian air strikes, have captured dozens of towns and villages in the country's north-west, which is the last stronghold of the opposition to President Assad.
More than half a million civilians, the vast majority of them women and children, have been displaced by the fighting, according to the United Nations.
The Turkish government, which backs the Syrian opposition and fears a massive influx of refugees, has called on Russia to bring an end to the offensive.
On Monday, eight Turkish military personnel monitoring the situation in Idlib were killed when they came under Syrian army artillery fire.
Fighting on Thursday was focused on the major opposition-held town of Saraqeb, which straddles the intersection of the strategic M4 and M5 highways.