Israeli warplanes bombed a convoy near Syria's border with Lebanon, sources told Reuters, apparently targeting weapons destined for Hezbollah in what some called a warning to Damascus not to arm Israel's Lebanese enemy.
Syrian state television accused Israel of bombing a military research center at Jamraya, between Damascus and the nearby border, but Syrian rebels disputed that, saying their forces had attacked the site. No source spoke of a second Israeli strike.
"The target was a truck loaded with weapons, heading from Syria to Lebanon," said one Western diplomat, echoing others who said the convoy's load may have included anti-aircraft missiles or long-range rockets. Several sources ruled out the presence in the convoy of chemical weapons, about which Israel has also raised concerns.
Diplomatic sources from three countries told Reuters that chemical weapons were believed to be stored at Jamraya, and that it was possible that the convoy was near the large site when it came under attack early on Wednesday.
However, there was no suggestion that the vehicles themselves had been carrying chemical weapons.
The raid followed warnings from Israel that it was ready to act to prevent the revolt against President Bashar al-Assad leading to Syria's chemical weapons and modern rockets reaching either his Hezbollah allies or his Islamist enemies.
A source among Syrian rebels said an air strike around dawn (0430 GMT) blasted a convoy near the border. "It attacked trucks carrying sophisticated weapons from the regime to Hezbollah," the source said, adding that it took place inside Syria.
Syrian state television said two people were killed in a dawn raid on the military site at Jamraya, which lies in the 25-km (15-mile) strip between Damascus and the Lebanese border. It described it as a scientific research centers "aimed at raising the level of resistance and self-defense".
It did not mention specific retaliation but said "these criminal acts" would not weaken Syria's support for Palestinians and other groups engaged in "resistance" to Israel.
Several rebel sources, however, including a commander in the Damascus area, accused the authorities of lying and said the only attacks at Jamraya had been mortar attacks by insurgents.
A regional security source said Israel's target was weaponry given by Assad's military to fellow Iranian ally Hezbollah.
"This episode boils down to a warning by Israel to Syria and Hezbollah not to engage in the transfer of sensitive weapons," the source said. "Assad knows his survival depends on his military capabilities and he would not want those capabilities neutralized by Israel - so the message is this kind of transfer is simply not worth it, neither for him nor Hezbollah."
With official secrecy shrouding the event, few details were corroborated by multiple sources. All those with knowledge of the events - from several countries - spoke anonymously.
There was no comment from Hezbollah or the Israeli government. Hezbollah's Al-Manar TV said only that Israeli warplanes had carried out "mock raids" over southern Lebanon on Wednesday night, close to the Syrian border.
Israel's ally the United States declined all comment. A Lebanese security source said its territory was not hit, though the army also reported a heavy presence of Israeli jets through the night after days of unusually frequent incursions.
Such a strike or strikes would fit Israel's policy of pre-emptive covert and overt action to curb Hezbollah and does not necessarily indicate a major escalation of the war in Syria. It does, however, indicate how the erosion of the Assad family's rule after 42 years is seen by Israel as posing a threat.
Israel this week echoed concerns in the United States about Syrian chemical weapons, but its officials say a more immediate worry is that the civil war could see weapons that are capable of denting its massive superiority in airpower and tanks reaching Hezbollah; the group fought Israel in 2006 and remains a more pressing threat than its Syrian and Iranian sponsors.
Israeli officials have said they feared Assad may be losing his grip on some chemical weapons, including around Damascus, to rebel groups which are also potentially hostile to Israel. U.S. and European security sources told Reuters they were confident that chemical weapons were not in the convoy which was bombed.
Wednesday's action could have been a rapid response to an opportunity. But a stream of Israeli comment on Syria in recent days may have been intended to limit surprise in world capitals.
The head of the Israeli air force said only hours before the attack that his corps, which has an array of the latest jet bombers, attack helicopters and unmanned drones at its disposal, was involved in a covert "campaign between wars".
"This campaign is 24/7, 365 days a year," Major-General Amir Eshel told a conference on Tuesday. "We are taking action to reduce the immediate threats, to create better conditions in which we will be able to win the wars, when they happen."
On Sunday, Prime Minister Benjamin Netanyahu, set for a new term after an election, told his cabinet that Iran and turmoil in Arab states meant Israel must be strong: "In the east, north and south, everything is in ferment, and we must be prepared, strong and determined in the face of all possible developments."
Israel's refusal to comment on Wednesday is usual in such cases; it has, for example, never admitted a 2007 air strike on a suspected Syrian nuclear site despite U.S. confirmation of it.
By not acknowledging that raid, Israel may have ensured that Assad did not feel obliged to retaliate. For 40 years, Syria has offered little but bellicose words against Israel.
(Additional reporting by Mariam Karouny and Oliver Holmes in Beirut, Myra MacDonald in London, Mark Hosenball in Washington and Reuters bureaux; Writing by Alastair Macdonald; Editing by David Stamp)