The Israeli air force has repeatedly demonstrated its superiority over Assad's outdated military, buzzing his residence in one famous instance to protest attacks by Syrian-backed militants and carrying out an airstrike on what the U.S. later said was an unfinished nuclear reactor.
Nonetheless, Israel worries the fall of Assad could have a range of grave consequences.
Officials have repeatedly warned that Assad may attack Israel in a final act of desperation if he fears his days are numbered. Israel also fears Syria could fall into the hands of Islamic extremists or descend into sectarian warfare.
Another lingering fear is that Syria's chemical weapons and missile could fall into the hands of its Lebanese ally, the Hezbollah guerrilla group, or other anti-Israel militants if Assad loses power. There are also concerns that Syria could become a staging ground for attacks by al-Qaida-linked groups battling Assad.
The aftermath of Egypt's revolution has provided Israel with reason to worry about its frontier region with Syria: Egypt's Sinai desert on Israel's southern border has turned even more lawless since longtime Egyptian President Hosni Mubarak was ousted in February 2011, and Islamic militants are now more easily able to use it as a launching ground for strikes against southern Israel.
On Sunday, Prime Minister Benjamin Netanyahu told his Cabinet that Israel is "closely monitoring" the border with Syria and is "ready for any development."
U.N. Secretary-General Ban Ki-moon said in a statement from his New York office that the shelling was reported in the U.N.-monitored zone between Israel and Syria, but no injuries to civilians or U.N. personnel were reported.
Ban called "for the utmost restraint" and urged Syria and Israel to uphold their cease-fire agreement and halt any exchange of fire.
The violence in Syria has killed more than 36,000 people in the uprising that began in March 2011. Hundreds of thousands have fled the fighting into neighboring Turkey, Jordan, Lebanon and Iraq. Another 11,000 escaped Friday into Turkey following the surge of fighting at Ras al-Ayn.
Ismail Aslan, the mayor of the nearby Turkish town of Ceylanpinar, said the number of refugees had slowed significantly Sunday. But Turkish soldiers at the border turned back some of the refugees who had arrived late last week and wanted to return to Ras al-Ayn, saying the area was not secure.
Activists reported clashes and shelling in different parts of Syria, killing almost 60 people, nearly two-thirds of them civilians.
The violence spread in most provinces around the country from Diaraa and Quneitra in the south to Idlib and Aleppo in the north to Homs and Hama in the center and Deir el-Zour in the east.
There also was fighting in Damascus and its suburbs, mostly in the capital's southern neighborhood of Tadamon.
In Qatar, Syrian activists said anti-government groups had reached a preliminary deal to form a new opposition leadership under pressure from the international community.
Ali Sadr el-Din Bayanouni, a former Syrian Muslim Brotherhood leader, said a broad agreement has been struck among the opposition factions to form a new group called the Syrian National Coalition for Opposition and Revolutionary Forces.
The new leadership, which was to choose a president later Sunday, will include representatives from the largest current opposition group, the Syrian National Council.
The Syrian opposition has been deeply divided for months despite repeated calls for them to unite.
The United States has become increasingly frustrated with the opposition's inability to form a common front and present a single conduit for foreign support.
Lauren E. Bohn in Jerusalem, Jamal Halaby in Amman, Jordan, Bassem Mroue in Beirut, Mehmet Guzel in Ceylanpinar, Turkey, and Abdullah Rebhy in Doha, Qatar, contributed reporting.
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