Israeli officials have confirmed its air force carried out an airstrike allegedly targeting a shipment of advanced missiles in Syria.
The officials said the target was not chemical weapons, but of "game changing" arms bound for the Lebanese militant group Hezbollah.
The strike took place overnight on Thursday into Friday, just as Israel was flying many warplanes over Lebanon.
The US earlier said it did not believe Israeli warplanes entered Syrian airspace to conduct the strikes.
Israeli Prime Minister Benjamin Netanyahu has repeatedly warned in recent weeks that his country would be prepared to take military action if chemical weapons or other arms were to reach Hezbollah.
Earlier this month, Israel admitted carrying out a January airstrike on a weapons convoy believed to be carrying anti-aircraft equipment in Syria thought to be en route to Hezbollah.
The confirmation of the strike came hours after Barack Obama refused to rule out options in dealing with the crisis in Syria but said he did not foresee sending in US ground troops.
Mr Obama, on a trip to Costa Rica, rules out US 'boots' on Syrian ground Speaking during a visit to Costa Rica, the US President said: "As a general rule, I don't rule things out as commander-in-chief because circumstances change and you want to make sure that I always have the full power of the United States at our disposal to meet American national security interests.
"Having said that, I do not foresee a scenario in which boots on the ground in Syria, American boots on the ground in Syria, would not only be good for America but also would be good for Syria."
Mr Obama said there was evidence that chemical weapons had been used in Syria, but that "we don't know when, where or how they were used".
But if "strong evidence" is found of such weapons being used by the regime of President Bashar al Assad, then "that is a game changer for us" because "there is a possibility that it lands in the hands of organisations like Hezbollah", Mr Obama said.
Mr Obama said if use of chemical weapons in Syria is confirmed, the US will present that evidence to the international community, because it concerns the entire world.
But he called for caution to avoid unintended consequences, saying: It's important for us to do it right."
Meanwhile, warnings of a new "large-scale massacre" in Syria have emerged following reports Mr Assad's troops bombarded Sunni areas near the city of Bania.
There have been reports of a 'large-scale massacre' in Baida The opposition National Coalition accused the regime of "war crimes and genocide", citing witness reports of civilians being stabbed to death.
"The Coalition calls on the Arab League and the United Nations to act rapidly to save the civilians of Baida, Banias and other villages across Syria," it said in a statement.
"Several sources in the village say at least 50 people were killed in summary executions and shelling in Baida village,"Rami Abdel Rahman, of the Syrian Observatory for Human Rights, told AFP news agency.
Syria's official Sana news agency said troops killed "terrorists" and seized arms in an operation targeting rebels.
Regular forces were supported by pro-regime "shabiha" militiamen, said the Observatory, which relies on a network of sources on the ground for its information.
The Banias region is predominantly Alawite, an offshoot of Shia Islam and the sect of Mr Assad, while the insurgents battling to topple his regime are mainly Sunni Muslims.
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