Israel’s defense minister, Ehud Barak, observed on Thursday that President Bashar al-Assad’s grip on power was slipping away.

“The disintegration is not abstract; it is real,” Mr. Barak said after a tour and debriefing with the local commander. “It is getting closer.”

The devolution in Syria, while welcome, presents a series of intensifying problems for Israel, its neighbor to the south. Israel’s leaders are growing concerned about Syria’s stockpiles of chemical weapons falling into the hands of rogue groups equally opposed to Israel; about the prospect of throngs of refugees appearing at the border; and about the Golan itself “turning into a lawless area where terror elements might also operate,” as Mr. Barak put it. There is concern that the collapse of the Syrian government could lead to a civil war in Lebanon.

Beyond that, the escalation in Syria, with the killing of several members of Mr. Assad’s inner circle, coming hours before a suicide attack on an Israeli tour bus in Bulgaria, only underscored how the Arab uprisings over the past 18 months have upended Israel’s strategic assessments about a neighborhood that it has traditionally viewed as hostile but stable.

No longer preoccupied with the Palestinians, Israel has now been confronted with a series of complex calculations. Should it strike Syria’s chemical weapons storehouses, as it did a Syrian nuclear reactor in 2007, or would that strengthen Mr. Assad’s hand by uniting the Arabs? Should it act alone against the Iranian nuclear program it sees as an existential threat, or let the United States plow ahead with diplomacy and sanctions? Should it act more aggressively against the military group Hezbollah in Lebanon? How should it navigate the shifting landscape in Egypt, where the new president hails from the Muslim Brotherhood?

“What you have in Syria is that the Middle East is coming apart; a new form of chaos is replacing what has existed,” said Dore Gold, a longtime diplomat who now runs the Jerusalem Center for Public Affairs. “The fundamentals you’re working with in the region are changing; you can’t just go back to the old discussions you might have had.

“Chaos is never an opportunity,” Mr. Gold added.

At the moment, the issue that looms largest may well be Syria’s arsenal of chemical and biological weapons. Over 40 years, Syria has amassed a stockpile, United States officials contend, of mustard gas, sarin and cyanide. In recent days, American and Israeli intelligence officials have said that Mr. Assad has been moving some of these weapons out of storage, apparently to keep them from falling into the hands of the rebels.

That has elevated concerns here that the weapons could fall into the hands of Israel’s enemies, including Islamist radicals who have taken up arms in the fight against Mr. Assad, or Hezbollah, which is increasingly worried over the potential fall of its patron.

“Israel will not sit idle,” said Danny Yatom, a former chief of the Mossad intelligence agency. “If we will have information that chemical agents or biological agents are about to fall into the hands of the Hezbollah, we will not spare any effort to stop it.”

But Shlomo Brom, a senior research fellow at the Institute for National Security Studies at Tel Aviv University, said that while the prospect of chemical weapons in the hands of terrorist groups is frightening, the threat may not be as dire as it seems. In order for the weapons to be used, Mr. Brom said, two substances must be combined in a certain way, and they must be deployed via aircraft.

“In many cases, the weapons are not really usable,” Mr. Brom said. “You need knowledge, you need systems, to use it.”

Mr. Assad’s Syria has remained a steadfast enemy of Israel. The two countries have no formal relations and are technically at war. Mr. Assad has been a provocateur whose support for Iran and Hezbollah is seen by Jerusalem as pernicious. But he is, as many said in interviews on Thursday, well known, part of the old Middle East that began to unravel last year with the fall of President Hosni Mubarak of Egypt. With Mr. Mubarak’s ouster, Israel has had to contend with rising lawlessness in Sinai and concerns over the fate of the two nations’ peace treaty.

There is no telling what the ramifications would be if Mr. Assad fell.

“Bashar kept the border quiet, and now it can be like in the case of Sinai, with chaos and terror,” said Eyal Zisser, chairman of the Middle East and African history department at Tel Aviv University. “Most Israelis do not care about the grievances and the aspirations of their neighbors, democracy, justice, prosperity. They care about their own security. That’s the way of the average Israeli, and as a result, his government.”

The Golan, a strategic plateau of about 450 square miles, is home to about 39,000 Israelis, and Mr. Barak warned on Thursday that the longer fighting continued in Syria, “the risk grows that the bloody residue left over between the sides” could turn it “into a lawless area where terrorists might operate.”

Still, several leading government officials and analysts here said Israel hardly seemed on a war footing, using the same words to describe its posture: “watching from the outside.” While the threat of a chaotic Syria — or, for that matter, a nuclear Iran or a desperate Hezbollah with dangerous weaponry — may seem most acute here, they said, Israel continues to count on international intervention.

“It’s not only an Israeli issue: if Qaeda or radical members will take control of nonconventional weapons, it might appear anywhere on the globe,” said Ilan Mizrahi, a former head of Israel’s National Security Council, and deputy chief of Mossad. “I do not think that we have to be the whip of God.”

The Bulgaria bombing only complicates the Syria question. “The Iranians would love to see Israel retaliate against Hezbollah in a limited way,” said Yoram Schweitzer, a senior research fellow at the Institute for National Security Studies. “It can create belligerent acts that could help Syria. The Iranians are very keen on helping Assad as his situation is getting worse and worse.”