ASSOCIATED PRESS JERUSALEM -- Israel was drawn into the fighting in neighboring Syria for the first time Sunday, firing warning shots across the border after an errant mortar shell landed near an Israeli military installation in the Golan Heights. While Israel appeared eager to calm the situation, its response was a potent reminder of how easily the Syrian civil war -- already spilling across borders with Turkey, Lebanon and Jordan -- could explode into a wider regional conflagration. Israeli officials threatened even tougher retaliation if attacks persist. They have feared that the instability in Syria over the past 19 months could spill across the border into Israel, particularly as President Bashar Assad's grip on power grows increasingly precarious. Israel has little love for Assad, who has provided refuge and support to Israel's bitterest enemies through the years. But the Syrian leader -- and his father before him -- have kept the frontier quiet for nearly four decades, providing a rare source of stability in the volatile region. The Israeli military said the mortar fire caused no injuries or damage at the post in the Golan Heights, which Israel captured from Syria in the 1967 Mideast war and then annexed. In recent weeks, incidents of errant fire from Syria have multiplied, leading Israel to warn that it holds Syria responsible. Israeli officials believe most of the fire has come from Syrian government forces, although they think it has been inadvertent and not been aimed at Israel. After responding to Sunday's mortar strike, the Israeli military moved quickly to defuse tensions. "We understand this was a mistake and was not meant to target Israel, and then that is why we fired a warning shot in retaliation," said Lt. Col. Avital Leibovich, a military spokeswoman. Defense officials said an anti-tank missile was fired, and there were no reports of casualties in Syria. The Israeli military also said it filed a complaint through United Nations forces operating in the area, stating that "fire emanating from Syria into Israel will not be tolerated and shall be responded to with severity." Israeli defense officials said the incident was not considered a serious military threat, but Israel felt the need to respond in order to set clear limits for the Syrians. Defense Minister Ehud Barak said Israeli defense forces have been instructed "to prevent the battles from spilling over into our territory." "Additional shelling into Israel from Syria will elicit a tougher response; exacting a higher price from Syria," Barak said. Nineteen months of fighting and the mounting chaos engulfing the Assad regime have already shaken the region, spilling into Lebanon, Turkey and Jordan. In new violence Sunday, Syrian army forces backed by helicopter gunships and artillery attacked a border area with Turkey after rebels captured a crossing point, activists said. The Syrian Observatory for Human Rights, a Britain-based activist group, said the Ras al-Ayn border area in Syria's northeast was "under siege" as dozens of rebels tried to hold onto the border crossing. The entry of Israel into the fighting would take the violence to a new level. Although Israel has a more powerful military, both countries have air forces and significant arsenals of tanks, missiles and other weapons. Israel is especially concerned about Syria's stockpile of chemical weapons. An Israeli war on Syria could also draw in Syria's ally, Hezbollah, further destabilizing the region. Hezbollah, which possesses tens of thousands of rockets and missiles, battled Israel to a stalemate during a monthlong war in 2006. On Israel's southern flank, Hamas militants in the Gaza Strip, who battled Israeli forces over the weekend, might also enter the fray. Yiftah Shapir, an analyst at the Institute for National Security Studies, a Tel Aviv think tank, said neither Israel nor Syria has an interest in allowing Sunday's hostilities to spin out of control. "I see the warning fire as an attempt to prevent any escalation," he said. "In Israel, no one wants a war with Syria or even an attempt to intervene in the events. The only thing that worries us is a spillover by this form or another. So I think it's a warning: 'Take care."' For Assad, a war with Israel could bring the end of his teetering regime. Israeli officials have said for months that it is only a matter of time before he is ousted. 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." 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.