With upheaval in Syria spreading and the crackdown by President Bashar Assad growing more violent, Israel has begun bracing for change in an authoritarian regime that has been a potent yet familiar enemy for four decades.

A shake-up in Syria would have implications beyond the border the two countries share. While Syria has not fought a direct war with Israel since 1973, it has cultivated relations with Israel's most bitter foes. A staunch Iranian ally, it backs Hezbollah in Lebanon and Hamas in the Gaza Strip.

These ties are suddenly in question as Assad faces the biggest challenge yet to his rule. Israeli officials now appear to believe that whether Assad survives, some sort of change in Syria is inevitable. For Israel, that will mean facing another wild card in a regional mix that has seen outwardly stable dictatorships quickly become volatile states in varying degrees of flux.

Any potential outcome in the power struggle holds "risks and opportunities" for Israel, said an Israeli official, speaking on condition of anonymity because of an order not to discuss the issue with the media.

Some in Israel believe changes in Syria's regime, or its disappearance altogether, could potentially weaken enemies such as Hamas and Hezbollah, which would work in Israel's favor. Others warn that the result could be anarchy or the strengthening of Islamic extremists.

All seem to agree that peace with Syria, which several Israeli governments have pursued without success, is off the table indefinitely.

Israeli officials are under strict instructions to remain silent on the events in Syria. This has made it difficult to gauge the official assessment.

But the Israeli official, who is privy to senior policy debates, said the government is closely following the developments in Syria and believes Assad is in a battle for survival.

The official said it is impossible to predict whether Assad will succeed in outwitting or overpowering his opponents. Israeli leaders are divided over whether his downfall would serve the country's interest, he said.

But he said officials believe irreversible change is under way. Even if Assad survives the challenge to his rule, the official said: "The Assad of the past is not the same Assad we will see in the future."

Israeli officials say that concerns that Assad may try to heat up hostilities with Israel to divert public outrage are unfounded, as he is currently too focused on his own domestic troubles.

Nonetheless, Israeli army officials said military commanders are holding briefings every few hours to monitor developments in Syria. Israel's military believes that Assad's deployment of the Syrian army to confront protesters shows just how serious the threat is. The officials spoke on condition of anonymity because they were discussing sensitive and confidential assessments.

The unrest sweeping the region has forced Israeli leaders to carefully calibrate their public statements. Israelis do not want to be seen as opposing the forces of freedom, but Israel has come to view moves toward democracy with suspicion, having watched Hamas and Hezbollah rise to power through internationally recognized elections.

In an appearance earlier this month on YouTube's World View Project, Prime Minister Benjamin Netanyahu offered support for democratic change in the Middle East but expressed concern that "democracy will be hijacked by radical regimes or militant Islamic regimes."

"We'd like to see everywhere, including in Syria, genuine reforms for democracy, genuine emergence of democracy," Netanyahu said. "That's no threat to any of us."

A widening crackdown by Assad's forces has killed more than 400 people across Syria since mid-March, according to Syrian rights groups.

Guy Bechor, a Mideast expert at the Herzliya Interdisciplinary Center, said months of unrest in Syria can be expected as Assad's ruling Alawite minority battles protesters in what he called a "zero-sum game for both sides."

"The assessment is that stability for Israel will continue in any case, as Syria will be busy with internal affairs for months or possibly years," Bechor said.

The Assad family has ruled Syria for four decades, constituting an unfriendly but stable presence along Israel's northeastern border.

Syria arms Hezbollah, the Lebanese guerrilla group that battled Israel in a monthlong war five summers ago, and also hosts the headquarters of the Palestinian Hamas, which has killed hundreds of Israelis in suicide bombings and rocket attacks. Last month, Israel's navy seized a ship carrying weapons that it said were sent by Iran and Syria to Hamas.

Regime change in Syria could be a blow to Israel's enemies, but could also usher in a successor that could be more extreme, Islamist and belligerent.

Syria has enforced decades of quiet along a shared frontier, and expressed willingness to make peace in return for the Golan Heights, which it lost to Israel in the 1967 Mideast war. Several rounds of talks have failed.

"There is always a tendency to stick with the status quo: quiet on the security front, quiet on the diplomatic front," said Eyal Zisser, a Syria expert at Tel Aviv University. "But there are those who say that with all due respect to quiet, Assad is causing more damage through Lebanon and Gaza."

The upheaval in Syria makes it unlikely that Israel will pursue a peace deal, but talks were not being discussed even before the recent protests, said Alon Liel, a former director general of Israel's Foreign Ministry and proponent of an Israeli-Syrian peace. "Renewal of talks was never on the agenda, and isn't on the agenda now," he said.

Some in Israel have seen the experience of Egypt, where Israeli ally Hosni Mubarak was ousted after 30 years in power, as a warning. Contenders for Egypt's presidency have taken a cooler line toward Israel and have suggested the peace treaty between the two countries would be reviewed and could be canceled.

Ties with Jordan, the only other Arab regime to have a peace agreement with Israel, could also be in jeopardy as regional unrest touches on the ruling monarchy. Ties with Turkey, another one-time ally, have soured in recent years as Turkey has tilted away from the West and toward the Islamic world.

"Syria cannot be seen alone. It's a part of everything else that is happening around us," Liel said. "Israel's isolation in the region is almost unprecedented."