Israeli planes have launched a strike inside Syria from Lebanese air space.
Unnamed US-based Israeli officials said the target was a shipment of arms destined for Lebanon's Hezbollah. The governments of Israel and Syria have not yet commented on the strike.
Israel launched a similar strike in January, when it also claimed to have targeted a Hezbollah-bound arms convoy.
Meanwhile, US President Barack Obama says he does not foresee sending US troops to tackle Syria's civil war.
Western intelligence agencies have raised concerns that the Syrian regime has used chemical weapons, something which the US has termed a "red line".
Mr Obama reaffirmed on Friday that clear evidence of chemical weapons would be a "game changer", but that any response would not be rushed.
Analysts say the US and its allies are discussing possible action including air strikes to enforce a no-fly zone, but Syria's ally Russia is strongly opposed to such measures.
Tens of thousands have been killed in two years of unrest in Syria.
On Saturday, activists said government forces had killed a number of people in the coastal town of Baniyas, days after more than 40 people were killed in a nearby village.
The activists said hundreds of people are trying to flee the area.
Reports of the air strike first emerged in US media reports quoting unnamed US officials.
Israeli officials later told journalists that the strike had taken place early on Friday.
While Israel rarely comments on specific operations, it has repeatedly said it would act if it felt Syrian weapons, conventional or chemical, were being transferred to militant groups in the region, especially Hezbollah, says the BBC's Wyre Davies in Jerusalem.
Lebanon's army had said in a statement saying Israeli warplanes had flown over Lebanese airspace for hours on Friday.
Without commenting directly on the reports, President Obama said on Saturday that the Israelis "justifiably have to guard against the transfer of advanced weaponry to terrorist organisations like Hezbollah".
Lebanese President Michel Suleiman denounced the flights and accused Israel of breaking international law.
The statements from Lebanon did not mention strikes against Syria.
The Syrian ambassador to the UN said he was not aware of any Israeli attack against his country.
Earlier this week, Israeli Defence Minister Moshe Yaalon acknowledged that Israel had launched an air strike in January against a target inside Syria.
He said that the transfer of sophisticated weapons to radical militant groups like Hezbollah was a red line, and Israel had acted when it was crossed.
During a visit to Costa Rica on Friday, Mr Obama told reporters that as a commander-in-chief he could rule nothing out "because circumstances change".
But he added he did not foresee a scenario in which "American boots on the ground in Syria" would be good for either America or Syria.
He also said he had already consulted Middle Eastern leaders and they agreed with him.
Mr Obama reiterated that there was evidence that chemical weapons had been used in Syria, but that "we don't know when, where or how".
He stressed that if strong evidence was found it would be "a game changer for us" because "there is a possibility that it (weapons) lands in the hands of organisations like Hezbollah" in neighbouring Lebanon.
Earlier this week, US Defence Secretary Chuck Hagel became the first senior US official to state publicly that Washington was reconsidering its opposition to supplying weapons to rebel forces.
"Arming the rebels - that's an option," he told reporters.
"You look at and rethink all options. It doesn't mean you do or you will. These are options that must be considered with the international community."
With no appetite for direct military intervention, many US officials increasingly feel that arming the rebels is now the least-worst option, the BBC's Kim Ghattas in Washington says.
US allies such as Qatar and Saudi Arabia are already providing weapons to various groups fighting Syrian President Bashar al-Assad's forces.
The pressure to act has intensified in recent days after emerging evidence that Syria has used chemical weapons such as the nerve agent sarin.
More than 70,000 people have been killed since fighting between forces of Syrian President Bashar al-Assad and rebels erupted in March 2011.