Israeli forces attacked a convoy on the Syrian-Lebanese border on Wednesday, sources told Reuters, after Israelis warned their Lebanese enemy Hezbollah against using chaos in Syria to acquire anti-aircraft missiles or chemical weapons.
"The target was a truck loaded with weapons, heading from Syria to Lebanon," said one Western diplomat, adding that the consignment seemed unlikely to have included chemical weapons.
A source among rebels fighting Syrian President Bashar Assad said an air strike around dawn (0430 GMT) blasted a convoy on a mountain track about 5 kilometers (3 miles) south of where the main Damascus-Beirut highway crosses the border. Its load probably included high-tech anti-aircraft and anti-tank missiles.
"It attacked trucks carrying sophisticated weapons from the regime to Hezbollah," the source said, adding that it took place inside Syria, though the border is poorly defined in the area.
A security official in the region also placed the attack on the Syrian side. A Lebanese security official denied any strike in Lebanon. It was not clear whether special forces took part.
The Israeli government declined comment on the issue.
Such a strike would fit its existing policy of pre-emptive covert and overt action to curb Iranian-backed Hezbollah and does not necessarily indicate a major escalation of the war in Syria. It does, however, indicate how the erosion of Assad's family rule after 42 years is seen by Israel as posing a threat.
Some analysts suggested Hezbollah was moving its own arms caches from stores in Syria, fearing rebels would overrun them.
Though Israel this week echoed concerns in the United States about Syrian chemical weapons, 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 from reaching Hezbollah; the group fought Israel in 2006 and remains a more pressing threat than its Syrian and Iranian sponsors.
Wednesday's strike could have been a rapid response to an opportunity. But a stream of Israeli comment on Syria in recent days was a reminder of a standing policy of pre-emptive strikes and may have been intended to limit surprise in world capitals.
The head of the Israeli air force said only hours before the strike 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.
In Lebanon, the army reported a heavy presence of Israeli jets over its territory throughout the night, following several days of increased incursions into Lebanese airspace. Israeli jets routinely fly and there have been unconfirmed reports in previous years of air strikes on Hezbollah arms shipments.
An Israeli attack inside Syria could be diplomatically provocative, particularly since Assad's Iranian ally said on Saturday that it would view such a strike as an attack on itself. Israel views Iran as its principal enemy and is engaged in a bitter confrontation over Tehran's nuclear program.
On Sunday, Prime Minister Binyamin Netanyahu, who is set for a new term after an election earlier this month, told his cabinet that both developments in Iran and turmoil in Arab states, notably Syria and Egypt, 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," he said.
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 US confirmation of it.
By not confirming 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.
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 wrong hands.
Interviewed on Wednesday, Shalom would not be drawn on whether Israeli forces had been in action in the north, instead describing the country as part of an international coalition seeking to stop spillover from Syria's two-year-old insurgency.
Recalling that US President Barack Obama had warned Assad of US action if his forces use 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."