Israel is getting increasingly jumpy as Syrian rebels, particularly the increasingly effective Islamists, steadily throttle the beleaguered Damascus regime.

Unusually heavy air force activity over Lebanon in recent days is raising suspicions Israel's preparing for airstrikes to ensure the Jewish state's security as Islamists advance into southern Syria close to the occupied zone in the Golan Heights.

U.N. peacekeeping sources in Lebanon say the Israelis have doubled their violations of Lebanese airspace this year compared to the equivalent period in 2012.

Israel began overflying Lebanon in May 2000 when its troops fled the border zone they had occupied since 1978. These flights, illegal under international law, have been primarily engaged in intelligence-gathering.

The marked increase in the number of aircraft involved, including unmanned spy drones, and their flight patterns over Hezbollah strongholds and suspected missile sites in recent days suggest Israel may be preparing for sizable offensive air operations against Hezbollah or Syria, or both.

The U.N. peacekeeping force reported a "very unusual" operation March 14 involving 25 Israeli F-15 and F-16 jets in four separate flights over Nabatiyeh in south Lebanon up to the Bekaa Valley, Hezbollah's heartland along the Syrian border, in the north.

U.N. personnel said the aircraft flew along north along Lebanon's Mediterranean coast before turning south just short of the Syrian port of Tartous, where the Russian navy maintains a base.

The Daily Star newspaper in Beirut surmised that "the hourlong mission could have been a training exercise to target Syrian coastal radar and air-defense systems." These include advanced Russian missiles acquired two years ago.

The Israelis' biggest fear is that the increasingly pressured Assad regime may resort to unleashing chemical weapons, most probably against the rebels but possibly against the Jewish state as well.

That alarm is heightened amid concerns Damascus has been transferring heavy weapons, including surface-to-surface and surface-to-air missiles. to Hezbollah in Lebanon and could supply chemical weapons as well.

Israel mounted a Jan. 30 airstrike that reportedly destroyed a weapons convoy leaving a Syrian army depot near the Lebanese border, widely seen as a warning to Damascus.

Hezbollah missile sites and other installations, including suspected command centers, were heavily blasted in a withering Israeli air blitz during the 34-day 2006 war.

But this time around, Hezbollah, which is backing longtime ally Assad, is concerned about being cut off from Iran and its arms supply line if the Damascus regime falls.

The Israelis say chemical weapons have been used in northern Syria, most likely by the Assad regime, although the United States and the United Nations haven't been able to confirm that.

But recent events have "highlighted Israel's concerns about the dangerous fallout from its crumbling neighbor," an analysis by the Washington Institute for Near East Policy, a U.S. think tank, said.

The ways Israel sees it, "Assad's readiness to use CW and cross a red line, if confirmed, is in keeping with his previous decision to cross an Israeli red line by providing strategic weapons to Hezbollah," the institute observed.

"Above all, Israel is concerned that the tumult could enable hostile elements -- jihadists in Syria or Hezbollah in Lebanon -- to acquire weapons from the Assad regime's huge strategic stockpile, whether chemical or conventional."

Meantime, the Syrian rebels are growing impatient with the United States and its European allies for not providing direct military support to them to fight Assad, or responding to the regime's use of strategic weapons.

This, the institute observed, has "led to growing resentment of Washington, a sentiment harvested by increasingly influential extremist groups in Syria."

The powerful jihadist groups, particularly the al-Nusra Front, are pushing into southern Syria, where they face Israel across the 1973 war cease-fire line on the Golan.

"The Israelis have shown they possess good intelligence on the security situation in Syria," noted Lebanese analyst Tony Badran," and that they will act on that intelligence when the situation calls for it.

The Jan. 30 airstrike "ought to serve as a message that Israel is watching closely and will hit anything it deems a threat.

"So far, this is being done through air power. However, one should not discount limited ground operations, with a very light footprint, down the road should the need arise."