United Nations Secretary-General Ban Ki-moon warned against escalating a fraught situation in Syria after Israel struck targets near Damascus on Sunday, targeting what its officials said were Iranian missiles bound for Hezbollah militants.
Israeli officials said the raid, the second in 48 hours, was not connected to Syria's civil war. It was aimed, rather, at stopping Lebanon's Hezbollah, an ally of Iran, acquiring weapons that could be used to strike Tel Aviv if Israel followed through on threats to attack Iranian nuclear facilities.
Iran denies Israeli and Western accusations that it is bent on acquiring atomic weapons - a long-running dispute that now threatens to intersect with the bloody strife in Syria.
The U.N. said Ban called on all sides "to act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict".
People had been woken in the Syrian capital by explosions that shook the ground and sent flames high into the night sky.
"Night turned into day," one man told Reuters from his home at Hameh, near one of the targets, the Jamraya military base.
The government of Syrian President Bashar al-Assad accused Israel of effectively helping al Qaeda Islamist "terrorists" and said the strikes "open the door to all possibilities".
Despite the angry rhetoric, Israeli officials said that, as after a similar attack in the same area in January, they were calculating Assad would not pick a fight with a well-armed neighbour while preoccupied with survival at home.
Two years of violence in which at least 70,000 Syrians have died have inflamed wider regional tensions between Shi'ite Muslim Iran and Sunni Arabs, some of them close Western allies, and left Western powers scrambling for an effective response.
Senior Republican Senator John McCain said on Sunday that the air strikes could add pressure on Washington to intervene in Syria, although President Barack Obama has said he has no plans to send ground troops.
Egypt, the most populous Arab state and flagship of the 2011 Arab Spring revolts, where elected Islamists have replaced a Western-backed autocrat, has no love for Assad, but it condemned the air strikes as a breach of international law that "made the situation more complicated".
Israel does not confirm such missions explicitly, but an Israeli official told Reuters on condition of anonymity that the strikes were carried out by its forces, as was an initial raid early on Friday.
A Western intelligence source told Reuters: "In last night's attack, as in the previous one, what was attacked were stores of Fateh-110 missiles that were in transit from Iran to Hezbollah."
Prime Minister Benjamin Netanyahu said his aim for Israel was to "guarantee its future" - language he has used to warn of a willingness to attack Iran's nuclear sites, even in defiance of U.S. advice, as well as to deny Hezbollah heavier weapons.
He later flew to China on a scheduled trip, projecting confidence that there would be no major escalation - although Israel has reinforced its anti-missile batteries in the north.
Syrian state television said bombing at a military research facility at Jamraya and two other sites caused "many civilian casualties and widespread damage", but it gave no details. The Jamraya compound was also targeted by Israel on Jan. 30.
Hezbollah's Al-Manar television showed a flattened building spread over the size of a football pitch, with smoke rising from rubble containing shell fragments. It did not identify it.
Obama defended Israel's right to block "terrorist organisations like Hezbollah" from acquiring weapons after Friday's raid.
A White House spokesman said on Sunday: "The president many times has talked about his view that Israel, as a sovereign government, has the right to take the actions they feel are necessary to protect their people."
It was unclear whether Israel had sought U.S. approval for the strikes, although the White House spokesman said: "The close coordination between the Obama administration, the United States of America, is ongoing with the Israeli government."
Obama has in recent years worked to hold back Netanyahu from making good on threats to hit facilities where he says Iran, despite its denials, is working to develop a nuclear weapon.
Iran has long backed Assad, whose Alawite minority is an offshoot of the Shi'ite Islam it practises. It denied the attack was on armaments for Lebanon.
Hezbollah, a Shi'ite movement that says it is defending Lebanon from Israeli aggression, declined immediate comment.