Israeli warplanes hit a weapons shipment in Syria headed for Lebanon's Hezbollah, media quoted US officials as saying, with a diplomatic source reporting Saturday that the strikes targeted arms stored at Damascus airport.
The news came as US President Barack Obama appeared to all but rule out deploying American troops to Syria, and as activists reported an exodus of Sunni residents of the 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 launched an overnight air strike on Thursday.
The United States does not believe Israeli warplanes entered Syrian airspace during the raid, it added.
Lebanon's army said pairs of Israeli planes entered Lebanese airspace three times 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 probably 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 students killed in the violence that has engulfed Syria since an uprising against his regime began in March 2011.
The Syrian Observatory for Human Rights watchdog said hundreds of families were fleeing Sunni districts in the northwest port of Banias.
The exodus comes after shelling on Sunni areas of the city on Friday and reports of a "large-scale massacre" in the Sunni village of Bayda on Thursday.
The Observatory said at least 50 people were killed in Bayda, many of them in summary executions.
Another 10 people were killed in the Banias district of Ras al-Nabaa on Friday, the group said, distributing grisly footage of dead bodies, including those of children.
Elsewhere, regime forces and Hezbollah fighters advanced towards the town of Qusayr in central Homs, with fighting around the area and air strikes on the town killing at least 16 people, 12 of them rebels, the Observatory said.
Rebel prime minister Ghassan Hitto, meanwhile, said he was close to naming an interim government, and warned against "political manoeuvring."
The opposition Syrian National Coalition umbrella group that elected him is divided over his nomination and the formation of an interim government, and several members have suspended their participation in the body.
Earlier, 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," he said in Costa Rica.
"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 probably used chemical weapons on his own people.
At least 70,000 people have been killed in Syria, according to the UN, including at least 36 people on Saturday, according to a preliminary toll from the Observatory.
The group said 122 people were killed in violence throughout the country on Friday -- 26 soldiers, 56 civilians and 40 rebels.