But the most immediate threat the Israelis perceive from that quarter is not Iranian-backed Hezbollah.
The Lebanese Shiite group has committed thousands of its troops to support the embattled Damascus regime, a key ally of Iran, and is unlikely at this time to want to open another front against the Jewish state, its traditional enemy.
The biggest danger the Israelis see right now is the growing number of Sunni jihadist fighters linked to al-Qaida who have become the main rebel force battling to topple the regime of President Bashar Assad.
The Israelis are only too happy to see these two enemies, Hezbollah and the jihadists, fighting each other. But the Jewish state cannot afford to let down its guard against a massive missile attack by Hezbollah militants or allow them to acquire advanced surface-to-air missile systems that would seriously challenge Israel's long-held mastery of Levantine skies.
Details of the airstrike carried out overnight Monday are sketchy, and there has been no official comment by Hezbollah, Israel or Lebanon. But it would be the sixth such attack the Israelis have mounted against weapons transfers to Hezbollah
Israel has not formally acknowledged any of the airstrikes that began Jan. 13, 2013, with the destruction of a truck convoy supposedly carrying Russian-made SA-17 anti-air missiles. The Israelis consider them a strategic game-changer if Hezbollah acquires them.
Senior officials have made little secret of their determination to do whatever it takes to ensure Hezbollah does not get its hands on such weapons.
The Israeli air force is generally believed to have also carried out three long-range strikes in the Red Sea and Sudan, a key supply route from Iran to Hezbollah, in 2012-13 against shipments of Iranian missiles purportedly en route to Hezbollah.
But the Lebanese National News Agency said the targets were near the town of Nabi Sheet in Lebanon's Bekaa Valley, Hezbollah's heartland along the Syrian border. It was the site of the January 2013 attack.
The previous airstrikes took place in Syrian airspace. If the most recent attack was against a target on Lebanese territory it would be the first reported since Hezbollah's 34-day war against Israel in July-August 2006.
The reported airstrike followed weekend remarks made by Israel's chief of staff, Lt. Gen Benny Gantz who warned during a visit to the northern front: "We're monitoring closely the transfer of all types of weapons to all fronts. Sometimes, in cases of necessity, something can happen."
Gantz said Israeli forces were in a "very high state of readiness" amid the growing perception in the Jewish state that the nature of the Syrian conflict has become intensely sectarian between Sunnis and Shiites, the cockpit of a similar confrontation in Iraq, Syria's eastern neighbor.
Israel, observed Middle East analyst Ian Black, "is increasingly alarmed about the disintegration of the country and the rise of jihadist-type groups in the uprising against Bashar Assad.
"Eventually, the fears is, they will turn their attention to Israel," he wrote in The Guardian daily of London.
"Hopes that the current regime in Damascus will be replaced by a democratic system that will preserve Syria intact have long faded.
"Few expect that the U.N.-brokered Geneva peace talks to achieve anything. Israel's principal involvement in the Syrian crisis has been to use its air power to prevent weapons transfers to Hezbollah -- a 'red line' that Binyamin Netanyahu has maintained.
"Half a dozen attacks -- none of them avowed -- have been in defense of Israel's strategic dominance, not direct intervention in the war."
Israeli analyst Ehud Yaari, currently with the Washington Institute for Near East Policy, noted Jan. 29 that for the Jewish state "preventing the southward expansion of extremist Islamist groups is becoming a larger priority in tackling the overall Syrian problem.
"If al-Qaida affiliates take charge of the regions bordering Israel and Jordan, new terrorist threats would arise, potentially exporting Syria's bloodshed to its neighbors.
"Such a development would give al-Qaida freedom of action over a vast area stretching from west of Baghdad to southern Syria. Put another way, the organization would have achieved a long-sought objective: a front with Israel."
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