Israeli warplanes targeted a Syrian weapons shipment headed for Lebanon's Hezbollah, media quoted US officials as saying, with a diplomatic source reporting Saturday strikes were launched to destroy arms stored at Damascus airport.
The news came as US President Barack Obama appeared to all but rule out deploying US troops to Syria, and as activists reported an exodus of Sunni residents of the Syrian city of Banias after a "massacre" in a nearby village on Thursday.
CNN said US and Western intelligence agencies were reviewing information suggesting Israel had conducted an air strike overnight on Thursday.
But the United States does not believe Israeli warplanes entered Syrian airspace to carry out a raid, it added.
Lebanon's army said pairs of Israeli airplanes entered Lebanese airspace on three occasions on Thursday night and stayed for two to three hours at a time.
NBC cited US officials as saying the primary target was believed to be a weapons shipment headed for Hezbollah, the Lebanese Shiite group closely allied with President Bashar al-Assad's regime.
One said the raid was likely tied to delivery systems for chemical weapons, but CNN cited officials as saying there was no reason to believe Israel had struck chemical weapons storage facilities.
A Syrian military source denied the raid had taken place at all, and an Israeli defence official would say only the Jewish state "was following the situation in Syria and Lebanon, with an emphasis on transferring chemical weapons and special arms".
But a diplomatic source in Lebanon told AFP that the operation destroyed surface-to-air missiles recently delivered by Russia that were being stored at Damascus airport.
On Friday, Syrian state news agency SANA reported that rebel forces had fired two rockets at the airport at dawn, hitting a kerosene tank.
If confirmed, the raid would mark the second time Israel has hit Syria this year after it implicitly admitted carrying out a January strike on weapons thought to be en route to Hezbollah.
Amid the speculation, Assad made a rare public appearance at the unveiling of a monument to Syrian students killed in the violence that has engulfed the country since an uprising against his regime began in March 2011.
In the south of the country, activists said hundreds of families were fleeing Sunni districts in Banias.
"Hundreds of families are fleeing Sunni neighbourhoods in Banias in fear of a new massacre," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
The exodus comes after shelling on Sunni areas of the city on Friday, and reports of a "large-scale massacre" in a nearby Sunni village on Thursday.
The Observatory said at least 50 people were killed in the village of Bayda, many of them in summary executions and shelling.
On Friday, the violence left at least nine people dead, Rahman said, including at least one child.
The opposition National Coalition denounced a "large-scale massacre" in Bayda, citing witness reports of civilians being stabbed to death.
State media said troops killed "terrorists" -- the regime term for insurgents -- in an operation targeting rebels.
Meanwhile, speaking in Costa Rica, Obama came close to ruling out deploying US troops to Syria.
"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," Obama said
"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."
Speculation has mounted that the Obama administration could reverse its opposition to arming the rebels after the White House said last week Assad had likely used chemical weapons on his people.
At least 70,000 people have been killed in Syria, according to the UN.
The Observatory said at least 122 people were killed in violence throughout the country on Friday, including 26 soldiers, 56 civilians and 40 rebels.