Syria on Thursday lodged a complaint with the United Nations over what it says was an Israeli air raid on a military research centre, as President Bashar al-Assad's allies rushed to denounce the strike.
In the face of the growing chorus of condemnation, Israel maintained a stony silence over Syria's claims, as well as over separate reports from security sources that its jets had struck a weapons convoy near the Lebanon border.
Syria said it complained to the United Nations, even though it is still technically at war with the Jewish state.
State news agency SANA said the foreign ministry's complaint evoked a 1974 disengagement agreement.
"The Ministry of Foreign Affairs has summoned Iqbal Singha, commander of the UN Observer Disengagement Observer Force... and informed him of an official protest over the Israeli violation of the disengagement agreement of 1974," the ministry said.
Reaction from close Damascus ally Iran was strident, with deputy foreign minister Hossein Amir Abdollahian warning of severe fallout.
Without elaborating, he said the "Zionist regime's attack on the outskirts of Damascus will have grave consequences," in remarks reported by the Isna news agency.
In the past, Iran has said that any Israeli attack on Syria would be considered an attack on the Islamic republic.
Russia's foreign ministry said it was "deeply concerned" but in its initial reaction said it was still trying to verify the Damascus allegations.
"If this information is confirmed, then we are dealing with unprovoked strikes against targets located on the territory of a sovereign state, which brazenly infringes on the UN Charter and is unacceptable, no matter the motive used for its justification," it said a statement in Moscow.
While Lebanese militia Hezbollah denounced "a new Zionist aggression," the movement which fought a devastating war with Israel in 2006 made no mention of revenge, prompting speculation it was likely to hold its fire -- for now.
In Cairo, Arab League chief Nabil al-Arabi condemned the "Israeli aggression," saying in a statement that it violated the territory of a sovereign Arab state.
Syria late on Wednesday accused Israel of launching a dawn strike on a military research centre in Jamraya, near Damascus.
"Israeli fighter jets violated our airspace... and carried out a direct strike on a scientific research centre in charge of raising our level of resistance and self-defence," the army general command said.
The warplanes entered Syria's airspace at low altitude and under the radar, the army said, adding that two site workers were killed.
"They... carried out an act of aggression, bombarding the site, causing large-scale material damage and destroying the building," state television quoted the military as saying.
Residents told AFP six rockets hit the complex, partly destroying it, causing a fire and killing two.
The Syrian army denied separate reports from security sources that an Israeli strike had targeted a weapons convoy from Syria near the border with Lebanon.
Israel has frequently warned that if Syria's stockpile of chemical weapons fell into the hands of the powerful Lebanese Shiite movement Hezbollah, a close ally of Assad's regime, it would be a casus belli.
It has also raised the alarm over long-range Scud missiles or other advanced weaponry, such as anti-aircraft systems and surface-to-surface missiles, being transferred to Hezbollah.
Israeli officials and the military refused to confirm or deny on Thursday any involvement in the alleged attack.
Commentators compared the modus operandi to a 2007 bombing raid on an undeclared Syrian nuclear facility at Al-Kibar, widely understood to be an Israeli strike but never acknowledged by the Jewish state.
On the political front, Syria's main opposition group was to meet Thursday in Cairo, a day after a surprise statement from its chief that he was willing to hold talks with regime officials, a Syrian National Coalition member said.
"This meeting was organised well before the Syrian National Coalition leader, Moaz al-Khatib, made his statement," SNC member Samir Nashar told AFP.
Khatib announced on Facebook on Wednesday that he was "ready for direct discussions with representatives of the Syrian regime in Cairo, Tunis or Istanbul."
He laid down as conditions the release of "160,000 detainees" and that the passports of exiled citizens be renewed in embassies abroad.
On the battlefront on Thursday, fierce clashes raged between soldiers and rebels on the southern outskirts of Damascus as regime tanks pounded the area, a watchdog group said.
The United Nations says a total of more than 60,000 people have been killed in the country's 22-month conflict.