Israeli jets bombed a convoy near Syria's border with Lebanon early on Wednesday, 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 centre, at Jamraya between the capital 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 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. However, there was no suggestion that the vehicles themselves had been carrying chemical weapons.
The overnight 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 a 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 centres "aimed at raising the level of resistance and self-defence".
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 neutralised 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 the Israeli government nor Hezbollah. Israel's ally the United States declined all comment. A Lebanese security source said its territory was not hit, though the army 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."
Jets over Lebanon
In Israel, where media operate under military censorship, broadcasters immediately relayed international reports of the strike. Channel Two television quoted what it called foreign sources saying the convoy was carrying anti-aircraft missiles.
Israeli jets routinely fly over Lebanon and there have been unconfirmed reports in previous years of strikes on Hezbollah arms shipments. An attack inside Syria could be diplomatically provocative, however, since Assad's Iranian ally said on Saturday that it would view any strike as an attack on itself.
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."
The Israeli military confirmed this week that it had lately deployed two batteries of its Iron Dome rocket-interceptor system to around the northern city of Haifa, which came under heavy Hezbollah missile fire during a brief war in 2006.
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. A failing Assad administration, some Israelis fear, might be tempted into more action, while Syria's Islamist rebels are also hostile to Israel and could present a threat if they seize heavier weapons.
Israeli Vice Premier Silvan Shalom said on Sunday that any sign that the Syrian army's grip on its presumed chemical weapons stocks was slipping could trigger Israeli intervention.
But Israeli sources said on Tuesday that Syria's advanced conventional weapons, much of it Russian-built hardware able to destroy Israeli planes and tanks, would represent as much of a threat to Israel as chemical arms in the hands of an enemy.
Hezbollah fighters and the Syrian army have close relations and, while Damascus may have been reluctant to hand over key parts of its own arsenal to its Lebanese allies, some analysts suggest that if Syrian or Hezbollah commanders fear hardware is about to fall into rebel hands they might try to move it across the border - possibly even without formal government approval.
On Wednesday, Israel's Shalom would not be drawn on whether Israeli forces had been in action in the north, but described the country as part of an international coalition seeking to stop spillover from Syria's two-year-old insurgency.
Recalling that President Barack Obama had warned Assad of U.S. action if his forces used chemical weapons, Shalom told Israel Radio: "The world, led by President Obama, who has said this more than once, is taking all possibilities into account.
"Any development ... in a negative direction would be something that needs stopping and prevention."
During the 2006 war in Lebanon, Israel's air forced faced little threat, though its navy was taken aback when a missile hit a ship. Israeli tanks suffered losses to rockets, and commanders are concerned Hezbollah may get better weaponry.
In what might have been a sign of seeking to reassure major powers, Israeli media reported this week that the country's national security adviser was despatched to Russia and military intelligence chief to the United States for consultations.
Shashank Joshi of the Royal United Services Institute (RUSI) in London saw any strike on Wednesday as intended to deliver a signal rather than heralding a major escalation from Israel.
"The Israelis are sending a message not just to Hezbollah but also to Assad's forces that they have no wish to get dragged in, but chemical weapons and certain types of missiles are a red line for them, and that regime forces ought to signal, in turn, to Hezbollah that they should proceed with caution," he said.
Worries about Syria and Hezbollah have sent Israelis lining up for government-issued gas masks in recent days. According to the Israel post office, which is handling distribution of the kits, demand roughly trebled this week.
"It looks like every kind of discourse on this or that security matter contributes to public vigilance," its deputy director Haim Azaki told Israel's Army Radio. "We have really seen a very significant jump in demand."
(Additional reporting by Dominic Evans in Beirut, Myra MacDonald in London, Mark Hosenball in Washington, Khaled Yacoub Oweis in Amman and Reuters bureaux; Writing by Alastair Macdonald; Editing by David Stamp)