DAMASCUS - Syria threatened on Thursday to retaliate over what it says was an Israeli air raid, as President Bashar al-Assad’s allies rushed to denounce the strike that threatened to take the conflict beyond Syria’s borders.Israel maintained a stony silence over Syria’s claims, as well as over separate reports that its jets had struck a weapons convoy near the Lebanon border. Syria’s foreign ministry said Israel “and the states that protect it” are responsible for the air strike, and “affirms Syria’s right to defend itself and its territory and sovereignty,” state news agency SANA reported.It called on “all the competent UN bodies to take the necessary steps given this grave Israeli violation, and to guarantee that it will not happen again.” Damascus’s ambassador to Lebanon, Ali Abdel Karim Ali, stressed Syria’s right to respond to “the Zionist aggression.” “The Israelis, and the United States behind them, along with their Arab and regional accomplices, realise that Syria, which defends its sovereignty and territory, may decide to respond by surprise to this aggression.”“It is up to the competent powers to choose the appropriate answer, and to determine the means and the place,” Ali added in remarks to Lebanese website Al-Ahad, which is close to the powerful group Hezbollah. Hezbollah denounced “a new Zionist aggression.” UN leader Ban Ki-moon is gravely concerned about reports of an Israeli air strike on Syria but cannot independently verify what happened, a spokesman said Thursday. Ban noted the reported strike with “grave concern,” said deputy UN spokesman Eduardo del Buey. “The secretary general calls on all concerned to prevent tensions or their escalation in the region.” 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 for Tel Aviv,” 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 was still trying to verify the Syrian 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.Late on Wednesday, Syria 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, saying two workers had been killed.The army denied separate reports citing security sources that an Israeli strike had targeted a weapons convoy from Syria near the border with Lebanon. Amid speculation a convoy might have been en route to supply Hezbollah, the White House warned Syria not to do so.While not confirming the targets of the Israeli raid, Ben Rhodes, a deputy national security adviser, said that might “further destabilise the region.” Meanwhile, the White House said Vice President Joe Biden will discuss the carnage in Syria in Munich on Saturday with Russian Foreign Minister Sergei Lavrov and Syrian opposition chief Moaz al-Khatib. Israel has frequently warned that if Syria’s stockpile of chemical weapons fell into Hezbollah hands, this 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 said on Wednesday he was “ready for direct discussions with representatives of the Syrian regime in Cairo, Tunis or Istanbul,” laying down conditions including the release of “160,000 detainees.”On the battlefront on Thursday, fierce clashes raged between soldiers and rebels on the southern outskirts of Damascus as regime tanks pounded the area, the Syrian Observatory for Human Rights said. At least 21 rebel fighters and 26 regime troops have been killed in less than two days in the northwestern Syrian province of Idlib, a watchdog said on Thursday. Large swathes of Idlib province, near the border with Turkey, have slipped out of army control and into rebel hands. While clashes raged in the Palestinian refugee camp of Yarmuk and in Tadamun district early on Thursday, the army shelled the nearby Al-Hajar al-Aswad neighbourhood later in the day. Thursday’s violence came a day after at least 139 people were killed in violence across Syria, among them 51 civilians, 46 rebel fighters and 42 soldiers, according to the Observatory. On Thursday, at least 37 people were killed in the country, according to a preliminary count. The UNsays more than 60,000 people have been killed in the country’s 22-month conflict.