Israeli forces carried out an air strike overnight on a weapons convoy from Syria near the Lebanese border, security sources told AFP on Wednesday, speaking on condition of anonymity.
The attack came after Israel expressed concerns that Damascus's stockpile of chemical weapons could fall into the hands of Lebanon's Shiite Hezbollah group, an ally of the Syrian regime, or other militant groups.
Israeli officials have said such that a transfer would be a casus belli and likely spark an Israeli attack.
Sources differed on whether the strike took place on Syrian or Lebanese territory.
"The Israeli air force blew up a convoy that had just crossed the border from Syria into Lebanon," one source said, adding that the convoy was believed to be carrying weapons, without specifying the type.
An Israeli military spokeswoman declined to comment on the report.
A second security source, speaking on condition of anonymity due to the sensitivity of the issue, also confirmed to AFP that Israeli warplanes had hit a convoy allegedly carrying weapons to Lebanon but said the incident occurred just inside Syria.
"It was an armed convoy travelling towards Lebanon but it was hit on the Syrian side of the border at around 2330 GMT," the source said.
Both sources reported a high level of "unusual" Israeli activity over Lebanese air space, which began on Tuesday evening and continued overnight.
The Lebanese army confirmed that Israeli warplanes entered Lebanese airspace up to 16 times between 9:30am (0730 GMT) Tuesday and 2:00am Wednesday.
"Every day there are Israeli overflights, but on Tuesday they were much more intense than usual," a Lebanese security source told AFP on condition of anonymity.
The incident occurred just days after Israel moved two batteries of its vaunted Iron Dome missile defence system to the north and at a time of rising fears that the conflict in Syria could see weapons leaking into Lebanon.
A former head of intelligence at Israel's Mossad spy service, Amnon Sofrin, said on Wednesday that the Jewish state "should make any effort to prevent any weapons systems of that kind (chemical) going out to terror organisations."
Speaking to reporters in Jerusalem before reports of the attack emerged, Sofrin said Israel was unlikely to carry out air strikes on chemical weapons stocks because of the environmental risks.
"When you go and attack a... chemical weapons depot, you're going to do unwarranted damage, because every part will leak out and can cause damage to many residents," he said.
"But if you know of a convoy leading these kind of (chemical) weapon systems from Syria to Lebanon, you can send a unit to the proper place and try to halt it" on the ground, he added.
On Monday, Israeli newspaper Maariv reported that Prime Minister Benjamin Netanyahu had "urgently dispatched" his national security adviser Yaakov Amidror to Russia to ask Moscow to use its influence in Syria to prevent the transfer of chemical weapons.