Israeli air force combat aircraft struck at least two targets in Syria twice within a 48h period from 3 May, according to Syrian sources.
The first attack was performed in the early hours of 3 May, while a second followed early on 5 May. Loud explosions were heard during the air strikes, while video footage released by opposition forces shows secondary explosions following both attacks.
Syrian opposition sources indicate the targets were storage facilities for Iranian-produced Fateh-110 surface-to-surface missiles that are alleged to have been brought to Syria on their way to being delivered to Hezbollah militants in Lebanon.
Syrian state media reported that Israeli missiles had hit a military and scientific research centre in Jamraya, near Damascus, and caused casualties. The regime's deputy foreign minister Faisal al-Miqdad said the Israeli air strikes represented "a declaration of war".
Israel went on high alert status after performing the strikes, which are believed to have been completed without facing any anti-aircraft fire. Airspace in the north of Israel has been closed to enable detection of potential aerial threats and the air force has been placed at a top level of alert.
Other measures taken on 5 May include air force deployment of two Iron Dome rocket interceptor batteries in northern Israel.
In recent months, Israel has said any transfer of "game-changing" weapons from Syria to Hezbollah in Lebanon will cross a "red line".
Israeli sources say the Iranians have developed an improved model of the medium-range Fatah-110 missile. Older examples of the weapon are already held by Hezbollah, and Israel believes the advanced version was on its way to the organisation's missile units.
Tehran has provided few details of the new model, but experts in Israel say Iranian industry is constantly trying to improve the type's guidance system.
The Fatah-110 is about 26.2ft (8m) long and weighs about 3t, including a 0.5t warhead. With a maximum range of 135nm (250km), the missile threatens Israeli territory all the way to the Beer-Sheba area in the Negev.
Israel has not commented on the reported air strikes.