UN chief Ban Ki-moon has appealed for restraint after Israeli air strikes on targets near Damascus which prompted Syrian officials to warn "missiles are ready" to retaliate.
The Israeli raids hit military sites outside the Syrian capital early Sunday, in the second such reported attack in 48 hours targeting weapons thought to be destined for the Lebanese militant group Hezbollah.
The strikes have raised new concerns of a regional spillover of Syria's conflict, which has killed more than 70,000 people since it erupted in March 2011.
"The secretary-general calls on all sides to exercise maximum calm and restraint, and to act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict," Ban's spokesman Martin Nesirky said.
"The secretary-general urges respect for national sovereignty and territorial integrity of all countries in the region, and adherence to all relevant Security Council resolutions," he added.
Ban spoke by telephone with Arab League chief Nabil al-Arabi, whose 22-member bloc demanded UN Security Council intervention to stop such Israeli attacks.
Egypt condemned the raids as a "violation" of international law, Britain warned of "increasing danger" to the Middle East, while the Syrian regime's main regional ally Iran said it would shorten the existence of the Jewish state.
The National Coalition, the war-torn country's umbrella opposition group, also condemned the attack while accusing President Bashar al-Assad's regime of complicity by weakening the army in its battle with Syria's people.
Damascus said in a letter to the Security Council that Israeli warplanes launched an "aggression" at 1:40 am on Sunday (2240 GMT Saturday) by firing missiles against three Syrian army positions.
A diplomatic source in Beirut told AFP the sites were the Jamraya military facility, a nearby weapons depot and an anti-aircraft unit in Sabura, west of the capital.
The letter said claims Syria was transferring anything were "unfounded" and accused Israel of coordinating with "terrorist groups" -- the regime term for rebels who have been fighting to oust Assad.
The official Al-Ikhbariyah television quoted unnamed sources as saying Syrian forces were ready to retaliate if new "violations" occur.
"The Syrian missiles are ready to strike specific targets in case of any (further) violations," it said quoting the unidentified sources.
Sunday's strike came about 48 hours after a reported Israeli raid on a weapons storage facility at Damascus airport.
Residents of the Damascus district of Dumar said Sunday's strike felt "like an earthquake" in their neighbourhood about six kilometres (four miles) away from the targeted area.
Video footage on YouTube appeared to show missiles lighting up clouds, blazing fires, and an explosion producing a massive orange fireball.
Israel reportedly targeted the Jamraya facility earlier this year, in a January 30 raid that Israeli officials have implicitly acknowledged.
The Jewish state has frequently warned it would act to stop the transfer of sophisticated arms or chemical weapons to Hezbollah, with which it fought a devastating war in 2006.
Hezbollah and Iran, Israel's regional arch-foes, have steadfastly backed the Assad regime in its war against the insurgency.
US President Barack Obama, speaking after the first reported attack this week, said Israel was justified in protecting itself.
"The Israelis justifiably have to guard against the transfer of advanced weaponry to terrorist organisations like Hezbollah," he said, without commenting directly on the strike.
"We coordinate closely with the Israelis, recognising that they are very close to Syria, they are very close to Lebanon."
Israel boosted security measures, deploying two batteries of the Iron Dome missile defence system to the north and closing airspace there until May 9, the military said.
A senior Israeli source said the air force was on "high alert" and media reports said security was boosted at Israeli embassies worldwide.
The strikes came after the Syrian Observatory for Human Rights reported the bodies of at least 62 people had been found on Saturday in a Sunni district of the port of Banias after a regime assault, and warned of fresh "massacres".
At least 35 people were killed in violence on Sunday, the Observatory said, adding rebels also seized part of the Minnigh military airport where a regime general was killed on Saturday.