JERUSALEM — Israel was drawn into the fighting in neighboring Syria for the first time Sunday, firing a warning shot across the border after an errant mortar shell landed near an Israeli military installation in the Golan Heights.
The action by Israeli soldiers marked the first such military engagement between Israel and Syria in 39 years.
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 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.
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 United States 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 missiles 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.
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.
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.