An Israeli F-16 fighter jet has crashed amid Syrian anti-aircraft fire after an offensive against Iranian targets in Syria, the Israeli military says.
The two pilots parachuted to safety before the crash in northern Israel. It is believed to be the first time Israel has lost a jet in the Syrian conflict.
Israel was carrying out strikes after the launch of an Iranian drone into Israel. The drone was intercepted.
Syria opened fire after an Israeli act of "aggression", state media said.
Israeli strikes in Syria are not unusual, the BBC's Middle East correspondent Tom Bateman says, but the loss of an Israeli fighter jet marks a serious escalation.
The Israeli military later said it was responding with a big attack on Iranian targets.
IDF spokesman Jonathan Conricus later tweeted: "The IDF is striking numerous (12) Syrian and Iranian military targets in Syria, in response to Iranian aggression. More to follow."
How did events unfold on Saturday morning?
In a statement, the Israeli military said "a combat helicopter successfully intercepted an Iranian UAV [unmanned aerial vehicle] that was launched from Syria and infiltrated Israel".
It said the drone was identified quickly and was "under surveillance until the interception".
The drone went down on Israeli territory and was "in our possession", Israel Defense Forces (IDF) spokesperson Brig Gen Ronen Manelis said.
The military said that in response the IDF "targeted Iranian targets in Syria". It said the mission deep inside Syrian territory was successfully completed.
The two pilots ejected and were later taken to hospital. One of them was "severely injured as a result of an emergency evacuation", the IDF said.
It was not clear whether the F-16 jet was hit by anti-aircraft fire or went down for other reasons.
Red alert sirens sounded in areas of northern Israel and the Israeli-occupied Golan Heights due to Syrian anti-aircraft fire.
Residents reported hearing a number of explosions and heavy aerial activity in the area near Israel's borders with Jordan and Syria.
Syrian state media quoted a military source as saying that the country's air defences opened fire in response to an Israeli act of "aggression" against a military base on Saturday, hitting "more than one plane".
What is the Iranian presence in Syria?
Iran - Israel's arch-enemy - is a key supporter of Syrian President Bashar al-Assad, whose troops have been fighting rebel groups since 2011.
Tehran has sent military advisers, volunteer militias and, reportedly, hundreds of fighters from its Quds Force, the overseas arm of the Iranian Revolutionary Guards Corps (IRGC).
It is also believed to have supplied thousands of tonnes of weaponry and munitions to help President Assad's forces and the pro-Iranian Lebanese Hezbollah movement, which is fighting on Syria's side.
Israel has accused Iran of seeking to establish military bases and missile factories in Syria, something Israel says it will not allow to happen.
Tehran has faced accusations that it is seeking to establish not just an arc of influence but a logistical land supply line from Iran through to Hezbollah in Lebanon.