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 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. Earlier Sunday, Defense Minister Ehud Barak warned, "If a shell falls, we will respond."
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.