US media reports suggest that Israeli warplanes have launched an air strike inside Syria.
The reports, quoting unnamed US officials, say the likely target was a weapons site. Some indicate the arms were to be sent to Lebanon’s Hezbollah.
There has been no confirmation from the Israeli or Syrian government.
Meanwhile, President Barack Obama says he does not foresee sending US troops to Syria, amid concerns over the possible use of chemical weapons there.
Speaking on a visit to Costa Rica, Mr Obama reaffirmed that clear evidence that the Syrian government had used chemical weapons would be a “game changer”, but that any response would not be rushed.
The US officials – speaking on condition of anonymity – say the Israeli air strike probably happened on Thursday or Friday, but the aircraft did not enter Syrian airspace.
An Israeli embassy spokesman in Washington declined to comment.
But the spokesman – quoted by Reuters news agency – added: “What we can say is that Israel is determined to prevent the transfer of chemical weapons or other game-changing weaponry by the Syrian regime to terrorists, especially to Hezbollah in Lebanon.”
The Syrian ambassador to the UN said he was not aware of any Israeli attack against his country.
Meanwhile, Lebanon’s National News Agency cited a Lebanese army communique as saying that “two Israeli warplanes breached the Lebanese airspace over the southern town of Rmeish” on Friday.
It said “the enemy warplanes flew in circular manner over all Lebanese regions, and then left”.
Earlier this week, Israeli Defence Minister Moshe Yaalon acknowledged that Israel had launched an airstrike 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 for Israel, and Israel had acted when it was crossed.
In Costa Rica, Mr Obama told reporters on Friday 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 with 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 gas 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.