Analysis: Strike adds complexity to tense region
Israel's reported air attack against a target or targets in Syria leaves many questions unanswered.
Was one target hit? News agency reports, citing US sources, speak of an arms convoy carrying sophisticated surface-to-air missile systems being attacked near to the Syria-Lebanon border.
On the other hand the Syrian government itself denies this, saying instead that a scientific research facility in the Damascus suburbs was attacked.
Some US reports also suggest that a building was hit. The Israelis themselves are saying nothing. Rarely was the fog of war murkier.
This air strike is the first time that Israel is believed to have hit a target in Syria since its 2007 raid on a suspected nuclear reactor - a mission that neither the Israelis nor the Syrians have ever confirmed.
If so it points to the potential for a dangerous new escalation in the Syrian crisis; Israel signalling that it is more than ready to strike out if what it sees as "red-lines" are crossed.
Given the chaos inside Syria a riposte from the Assad regime itself is viewed by experts as unlikely. But Syria's ally, Hezbollah, for whom the arms were supposedly destined, may seek some way to respond.
The suggestion from US reports is that the air strike was an attempt to prevent the Syrian authorities handing over modern air defence systems to Hezbollah fighters in Lebanon. The weapon in question is thought to be the SA-17 - a modern Russian system which comprises four missiles on a tracked launcher, that also carries an associated radar system.
This would be a significant enhancement of Hezbollah's capabilities potentially limiting Israel's freedom of operation in Lebanon's skies.
The Lebanese authorities clearly object to Israel's almost routine infringement of their air-space but have very little ability to do much about it. The SA-17 could change that.
The air strike or strikes underscores Israel's growing alarm at what is happening inside Syria.
While a good share of Israel's and indeed Washington's attention is taken up by fears about Syria's chemical arsenal falling into the wrong hands, this latest air strike or strikes underscores Israel's equal worry about sophisticated conventional weapons being passed to Hezbollah.
Some four years ago the then Israeli government of Prime Minister Ehud Olmert warned that it would not tolerate what it called "game-changing" weapons being transferred to Hezbollah.
This included sophisticated anti-aircraft missiles, anti-shipping missiles, or long-range ground-to-ground missiles.
This Israeli operation can thus be seen as in one sense pre-emptive, but also as a warning to the Syrian authorities and to Hezbollah.
Why might the embattled Assad regime pass such weapons to its Lebanese allies?
Syria has been covertly arming Hezbollah for many years. Hezbollah, along with Iran, is among the few friends the Syrian leadership has left.
President Assad may be slowly losing his fight for survival in Syria but the demise of his regime is not yet a foregone conclusion.
He may believe that something will survive and having a well-armed ally in Lebanon may suit his longer-term strategic goals, if indeed there is "a longer-term" for the Assad regime.
Hezbollah - like Israel - clearly believes that there will be a new round of fighting between them.
It wants to bolster its already sizeable arsenal, especially in areas like air defence where it remains relatively weak.
Quite how it may respond is unclear. Last July's attack on an airport bus carrying Israeli tourists in Bulgaria suggests that if there is to be a response it might be indirect - against Israeli or Jewish targets abroad, rather than across Lebanon's own frontier with Israel.
Either way the situation in the region has become even more complex; the ramifications of the Syrian crisis thrown ever more starkly into relief.