The likely downfall of Bashar al-Assad could open new and dangerous fronts for its southern neighbour Israel, experts say, even if it weakens Hezbollah, the Syrian president's Lebanese ally.
While nobody can predict when or how 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.
Bulgaria said on Tuesday that Hezbollah was behind a deadly July bus bomb attack in the country that killed five Israeli tourists and one Bulgarian.
Interior Minister Tsvetan Tsvetanov told reporters investigators that two people involved in the attack "belonged to the military wing of Hezbollah."
Israeli Prime Minister Benjamin Netanyahu said the Bulgarian finding should push the EU to draw the "necessary conclusions" about the Shiite group, a reference to Israel's longstanding demand that it be placed on a terror watch list.
It "was an attack on European soil, against a member state of the European Union," he said. "We hope that the Europeans will draw the necessary conclusions about the true nature of Hezbollah."
He said it was "one in a series of terror attacks planned and carried out by Hezbollah and Iran," adding that the two Shiite forces supported "the murderous... regime in Syria."
Tehran denies any involvement in the Burgas attack.
Referring to an Israeli air strike inside Syria, the security official said that the Jewish state, along with Syria's neighbours and many Western nations, was concerned that Damascus's chemical weapons could fall into Hezbollah's hands.
"That is a red line I don't see Israel letting them cross," he said.
Last Wednesday's air strike targeted a military complex near Damascus a US official said contained 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."
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."