Israeli forces attacked a convoy on the Syrian-Lebanese border overnight, a Western diplomat and regional security sources said on Wednesday, as concern has grown in the Jewish state over the fate of Syrian chemical and advanced conventional weapons.

The sources, four in total, all of whom declined to be named because of the sensitivity of the issue, had no further information about what the vehicles may have been carrying, what forces were used or where precisely the attack happened.

In the run-up to the raid, Israeli officials have been warning very publicly of a threat of high-tech anti-aircraft and anti-tank missiles reaching Israel's enemies in the Lebanese militant group Hezbollah from Syria. They have also echoed U.S. concerns about Syria's presumed chemical weapons arsenal.

The Lebanese army reported a heavy presence of Israeli jets over its territory throughout the night.

"There was definitely a hit in the border area," one security source said. A Western diplomat in the region who asked about the strike said "something has happened", without elaborating.

An activist in Syria who works with a network of opposition groups around the country said that she had heard of a strike in southern Syria from her colleagues but could not confirm it. A strike just inside Lebanon would appear a less diplomatically explosive option for Israel to avoid provoking Syrian ally Iran.

Israeli Vice Premier Silvan Shalom said on Sunday that any sign that Syria's grip on its chemical weapons was slipping, as President Bashar al-Assad fights rebels trying to overthrow him, could trigger Israeli intervention.

Israeli sources said on Tuesday that Syria's advanced conventional weapons would represent as much of a threat to Israel as its chemical arms should they fall into the hands of Islamist rebels or Hezbollah guerrillas based in Lebanon.

Interviewed on Wednesday, Shalom would not be drawn on whether Israel was operating on its northern front, instead describing the country as part of an international coalition seeking to stop spillover from Syria's two-year-old insurgency.

"The entire world has said more than once that it takes developments in Syria very seriously, developments which can be in negative directions," he told Israel Radio, recalling that President Barack Obama has warned Syrian President Bashar al-Assad of U.S. action if his forces use chemical weapons.

"The world, led by President Obama who has said this more than once, is taking all possibilities into account," Shalom added. "And of course any development which is a development in a negative direction would be something that needs stopping and prevention."

BORDER STRIKE

Whether the strike took place within Syrian territory, or over the border in Lebanon, could affect any escalation from the incident. Iran, Israel's arch-foe and one of Damascus's few allies, said on Saturday it would consider any attack on Syria as an attack on itself. During and since Israel's 2006 war with Hezbollah, there have been unconfirmed reports of Israeli strikes on convoys just after they entered Lebanon from Syria.

Israel has long made clear it claims a right to act preemptively against enemy capabilities. Alluding to this, air force chief Major-General Amir Eshel on Tuesday said his corps was involved in a covert and far-flung "campaign between wars".

"This campaign is 24/7, 365 days a year," Eshel told an international conference. "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."

He did not elaborate on any operations, but did single out the threat Israel saw from Syria's arsenal, calling it "huge, part of it state-of-the-art, part of it unconventional".

Israel fought an inconclusive war in Lebanon with Iranian-backed Hezbollah in 2006. Its aircraft then faced little threat, though its navy was taken aback when a cruise missile hit a ship off the Lebanese coast. Israeli tanks suffered losses to rockets and commanders are concerned Hezbollah may get better weaponry.

Israeli jets regularly enter Lebanese airspace, but its forces have been more discreet about Syrian incursions.

Israel's bombing of a suspected Syrian nuclear reactor in 2007, though revealed by then U.S. President George W. Bush and Vice President Dick Cheney, is still not formally acknowledged by the Israelis.

According to Bush, then Israeli Prime Minister Ehud Olmert sought to keep the matter quiet so as to reduce the risk of Assad feeling public pressure to retaliate. Syria and Israel are technically at war but have not exchanged fire in a significant way in decades.

A U.N. force sits on the line, north of the Israeli-occupied Golan Heights, where a ceasefire ended their last war in 1973.

Israeli media reported this week that the country's national security adviser, Yaakov Amidror, was sent to Russia and its military intelligence chief Major-General Aviv Kochavi to the United States for consultations.

Shashank Joshi of the Royal United Services Institute (RUSI) in London said that there are indications that Hezbollah is training near chemical weapons sites in Syria, with which the Shi'ite Lebanese militia has historically had a strong alliance.

"We also know that (Syria's) use of tactical ballistic missiles has been escalating - presumably as air power becomes harder to use in contested areas, and rebels seize larger targets like bases that are amenable to missile attack," he said.

Worries about Syria and Hezbollah have sent Israelis lining up for government-issued gas masks. 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."