While nobody can predict when or how Bashar al-Assad's regime could crumble, such a scenario "would dramatically affect Israel," a senior security official speaking on condition of anonymity told AFP.
"Until now, the border with Syria has been the quietest of all our borders," he said.
The Syrians "took care to secure it, and focused their terrorist energy on their proxy, Hezbollah," the powerful Shiite group against which Israel fought a devastating 2006 war.
Any change in Syria could make "radical elements" operating in the frontier area freer to move, he said. "The border could become hot. We could be facing infiltration attempts and shootings." But Israel, along with Syria's neighbours and many Western nations, was more concerned that Syria's chemical weapons could reach "the wrong hands," the official said, referring to Hezbollah.
"That is a red line I don't see Israel letting them cross," he said, pointing to a reported air strike by Israeli jets last Wednesday on a military complex near Damascus which a US official said targeted surface-to-air missiles and an adjacent military complex believed to house chemical agents.
Damascus blamed Israel and has threatened to retaliate, and Syria's close ally Iran warned the attack would have "grave consequences" and that the "Zionist entity" would regret its aggression against Syria
Israel remained tight-lipped on the topic till Sunday, when Defence Minister Ehud Barak hinted at Israeli responsibility.
"It's another proof that when we say something we mean it," Barak told reporters at a security conference in Germany.
"We say that we don't think that it should be allowable to bring advanced weapon systems into Lebanon, the Hezbollah, from Syria, when Assad falls."
In a July interview, Barak was more explicit about what Israel perceived as a threat serious enough to justify Israeli military intervention, saying it believed Hezbollah would try to lay its hands on Syria's advanced anti-aircraft systems, surface-to-surface missiles "or elements of chemical weapons."
"I've ordered the army to prepare in such a way that if situations arise that will force us to consider action, we will be able to consider it," Barak said at the time.
According to former head of intelligence at Israel's Mossad spy service, Amnon Sofrin, while jihadist groups active in Syria could "take take advantage of the situation and operate against Israel," at this stage it is only Hezbollah that could mount chemical warheads on long-range missiles in their possession.
To deal with threats on the ground, Israel is upgrading the old security fence along its armistice line with Syria, and work is expected to be completed by the end of the year.
The new fence will be similar to one Israel has nearly completed on its Egyptian frontier. On the northern front, Israel is also considering declaring a buffer zone on the Syrian side of the fence, to prevent enemies from approaching.
Sofrin said there was also a danger that Assad might strike out at Israel in the final throes of his regime, bequeathing to Hezbollah "capability to hit Israel very bad."
But along with the concern over what the future instability within Syria may hold for Israel, Assad's eventual downfall would deal a "serious, though not lethal blow to Hezbollah," which Israel considers a threat more grave than various militant groups active in Syria, the senior security official told AFP.
"There is no doubt that the very falling of this central link in the Iranian array is a blow to Iran and Hezbollah, and something Iran is doing everything to prevent."