As Syria's Assad regime buckles under mass protests for reform and democracy, neighboring Israel is watching with unease.

True, the downfall of Syrian President Bashar al-Assad would ostensibly remove a key player in the Iranian-led alliance threatening the Jewish state on several fronts. But Syria under Mr. Assad has been a stable neighbor and maintained a regional balance that officials and analysts fear could crumble – providing an opening for hard-line Islamist groups.

"I prefer the political extremism of Assad over religious extremism," says Ayoub Kara, a parliament member from Prime Minister Benjamin Netanyahu's Likud party. "We don’t want religious extremism on the border."

Syria 101: 4 attributes of Assad's authoritarian regime

Two worst-case scenarios envision a boost for groups considered Islamic radicals. In one, Iran could gain greater influence in post-Assad Syria. In the second, contradictory scenario, the Syrian branch of the Muslim Brotherhood could control a new government.

While most analysts agree that the fall of Assad’s regime would remove a reliable ally of Iran, the Islamic Republic might use that power vacuum to forge a closer bridge to Hezbollah or gain sway over a fledgling Syrian ruler. And even the weakening of Assad's rule could give Iran an opportunity to expand its influence in Syria, by propping up Assad.

Israel is also afraid that if Syria’s Sunni majority were to replace the Alawite minority now in charge, it would give the Syrian branch of the Muslim Brotherhood a dominant role in the country. Even if the Sunni leadership were secular, analysts in Israel said they are likely to take even more of a hard line against Israel because of historic ties to Sunni Muslims in the Palestinian territories.

"Assad is definitely an enemy who helps Hamas and Hezbollah. But the disintegration is frightening," says Alon Liel, a former managing director of the Israeli foreign ministry who has advocated in the past for Israel-Syrian peace talks. "There is no one opposition group that can take control of Syria. It’s quite a mess."

Syria's Assad: A stable neighbor

In the past three decades, Israel and Syria have fought three wars with each other and another by proxy in Lebanon. Since then, Israel has accused Syria of sponsoring low-level violence in third party countries that occasionally flares up into a limited conflict, like Israel’s war with Hezbollah in 2006 and with Hamas in 2008-09.

During the same period, three rounds of peace talks have failed.

Giora Eiland, a former national security adviser under former Prime Minister Ariel Sharon, said Mr. Sharon once slammed an Israeli who suggested that regime change in Syria. "Sharon said, 'Are you crazy?' " he recalls. "The best for the time being, is having a Bashar Assad who is fighting for his legitimacy.' "

To be sure, Syria provides support for Hezbollah on the Israel-Lebanon border, and for Hamas on Israel’s border with the Gaza Strip. Both groups have fired rockets into Israel.