Israel's military strike on a target inside Syria has provoked a furious reaction, with Russia accusing the Government in Jerusalem of a "gross violation of the UN charter" and Syria threatening a "surprise" attack in retaliation.
Syria has summoned the head of the United Nations' observer force in the Golan Heights to complain about the strike, which saw Israeli jets blow up what United States officials claim was a convoy of SA-17 anti-aircraft missiles near the border with Lebanon.
Syrian state TV quoted the Foreign Ministry as saying: "Syria holds Israel and those who protect it in the Security Council fully responsible for the results of this aggression and affirms its right to defend itself, its land and sovereignty."
Syria's ambassador to Lebanon, Ali Abdul-Karim Ali, said the Assad regime had "the option and the capacity" to hit back, while Iran's Deputy Foreign Minister Hossein Amir Abdollahian warned of "significant implications" for the coastal city of Tel Aviv. Lebanese militant group Hizbollah described the attack as an act of "barbaric aggression".
What happened is disputed, with Syria claiming that the strike hit the Jamraya military research facility near Damascus, killing two people. This version has been questioned, particularly as state television has not shown footage of damage to the site.
Western sources claim the convoy was attacked en route to Lebanon, implying the weapons were intended for Hizbollah. Russia - a staunch ally of Syria - said: "If [the strike] is confirmed, then we are dealing with unprovoked attacks on the territory of a sovereign country, which blatantly violates the UN Charter and is unacceptable, no matter the motives to justify it."
Israel's Prime Minister Benjamin Netanyahu and other senior figures have repeatedly voiced their concerns about Syria's stockpile of chemical weapons - specifically Sarin gas and the VX nerve agent - being transferred to Hizbollah for use against the Jewish state.
In 2007, Israeli planes bombed a site in Deir Ezzor, Syria, which the UN had said it suspected of being a nuclear facility, while four years earlier it targeted a camp suspected of training Palestinian militants on Syrian soil.
"If Israel did carry out [Tuesday's] attack, the only reason can be that it felt if it didn't do so, the threat would have been a lot worse," said Miri Eison, a former deputy head of the combat intelligence corps in the Israeli army.
"The Israeli policy is never to comment on these things - it leaves manoeuvrability on all sides. It certainly seems as though Russia doesn't want to talk about the weapons it has transferred to Syria, and whether it would allow those weapons to be moved on to groups like Hizbollah."
Other sources close to the Israeli military echoed those fears. Dan Harel, a former deputy chief of staff of the Israeli army, said: "The Russians are terrified about Western powers seeing attacks on Syria, and using them as a precedent to intervene themselves. When we fought the war in 2006, we found all sorts of advanced weapons under Hizbollah's control in Lebanon - all from Russia. The Russians didn't have much to say, but here it is again, SA-17 missiles - they have maintained the supply of cutting-edge weaponry to Syria and this weaponry is making its way to Lebanon."
Syria's contention that Israel's target was Jamraya - a frequent target - may be an attempt to associate the rebels with Israel in the hope of stoking anti-Israeli sentiment among Assad's supporters. The attack comes at a time when neither President Assad nor Hizbollah is likely to want to engage in a major conflagration, although analysts say the action paves the way for a dangerous new escalation in an already protracted war.
Analysts fear the conflict could now spread into Lebanon and Israel. Fawaz Gerges, a professor of Middle East politics and international relations at the LSE, said: "Assad will certainly try to milk the attack - it allows him to present himself as a defender of the nation.
"[But] Assad is in terrible trouble and in many ways he can't afford not to retaliate."