During a tour of the Golan Heights, Defense Minister Ehud Barak gave a scathing assessment of Syrian President Bashar Assad's forces and said Israel will remain "vigilant and alert."
"Almost all of the villages, from the foot of this ridge to the very top, are already in the hands of the Syrian rebels," said Barak, who was accompanied by Prime Minister Benjamin Netanyahu. "The Syrian army is displaying ever-diminishing efficiency."
The civil war in Syria has renewed tensions over the Golan Heights, a strategic plateau that Israel captured from Syria in 1967. Despite hostility between the two countries, Syria has been careful to keep the border quiet since the 1973 Mideast war.
But in recent days, Israeli troops have fired into Syria twice after apparently stray mortar shells flew into Israel-held territory. On Wednesday, an Associated Press journalist said an Israeli helicopter was patrolling the border area, and gunfire could be heard. The source of the gunfire was not immediately clear.
While it is widely believed that Assad does not want to pick a fight with Israel, there are fears the embattled Syrian leader may try to draw Israel into the fighting in a bout of desperation. Israeli officials believe it is only a matter of time before Syrian rebels topple the longtime leader.
Israeli political scientist Dore Gold, an informal adviser to Netanyahu, said it's difficult to assess whether Israel is better off with rebels in control along the border.
"The forces fighting the Assad government are made up of diverse elements. And to make a judgment whether Israel should be more or less worried, that would require having a very precise picture of what's going on there, which we don't," he said. "But it's no secret that among the Syrian rebels are forces that identify with al-Qaida, and are a cause of concern."
A buffer zone lines the Israeli border with Syria. Beyond the border on the Syrian side is a 75-kilometer (46-mile) stretch where no military forces other than U.N. forces are permitted.
Israeli military officials said Barak's assessment depicted a situation that is not entirely new, and that rebels have held those villages for several weeks. It was not clear how many villages the rebels hold along the Golan Heights, which is about 40 miles (65 kilometers) from the Syrian capital of Damascus.
The officials, who spoke on condition of anonymity because they were not allowed to discuss the sensitive information, said the situation is dynamic and could change easily, with the villages returning to Assad's hands.
Israeli experts said nothing prevents Assad's forces from entering the villages and retaking them, even ones in the U.N. zone.
"Just like any other place, it is a battleground between the army and the rebels," said Itamar Rabinovich, the former chief Israeli negotiator with Syria.
He said Israel would likely continue to remain on the sidelines of the fighting because Israeli officials believe Assad will eventually fall and that any support for rebels would backfire.
But privately, "Israel is rooting for the right kind of insurgents," he said, ones who follow a moderate line and have no links to Islamist extremist groups.
Moshe Maoz, professor emeritus at Jerusalem's Hebrew University, said the exchange of fire this week was "based on a mistake," and that if such incidents continued, they would be infrequent.
"The Syrian army doesn't have any interest in provoking Israel," he said. "Syria has enough problems."
The violence in Syria, which has killed more than 36,000 people since the uprising against Assad began in March 2011, threatens to inflame an already combustible region. The fighting already has already spilled into Lebanon, Jordan and Turkey.
On Wednesday, Syrian troops used aircraft and artillery to try to dislodge rebels from a town next to the border with Turkey, as Ankara warned it would retaliate against any airspace violations.
An AP journalist in the Turkish town of Ceylanpinar saw Syrian airstrikes in the adjacent Syrian town of Ras al-Ayn, where rebels say they have ousted troops loyal to Assad.
Deadly airstrikes began several days ago, and many casualties were taken to Turkey for treatment. Local officials said as many as 30 people have died since Monday. The journalist also saw Syrian forces shelling a wooded area near Ras al-Ayn from where rebels had been firing.
Hundreds of thousands of people have fled the fighting in Syria into neighboring Turkey, Jordan, Lebanon and Iraq.
Another 11,000 escaped to Turkey last week following the surge of fighting at Ras al-Ayn, which is located in the northeastern Syrian province of al-Hasaka, an oil-producing region where the population is mostly Kurdish.
The proximity of the fighting to Turkey has raised fears of an escalation.
Turkish media, including the Anadolu news agency, said several villages west of Ceylanpinar have been evacuated to protect residents from any spillover of the fighting in Syria. About 1,000 people left Mursitpinar, 110 miles (180 kilometers) from Ceylanpinar, after an appeal from the loudspeakers of local mosques.
Ismet Yilmaz, Turkey's defense minister, indicated that military force would be used in response to any incursions by Syrian aircraft. Last month, Turkish artillery fired on targets in Syria after Syrian shells landed inside Turkey and killed several civilians.
"The necessary response will be given to Syrian planes and helicopters that violate our border," Yilmaz said.
A Turkish official in Ceylanpinar said the sound of shelling was heard through the night. Two rocket-propelled grenades hit houses on the Turkish side, but there were no injuries, according to the official, who spoke on condition of anonymity because he is barred by rules from being quoted by name.
The official later said a dozen wounded Syrians had been brought across the border, and one died during treatment. The official cited contacts in Ras al-Ayn as saying Syrian forces had entered the town.
A convoy of seven white jeeps and a truck was seen near the Syrian town, but it was unclear who was in the vehicles. On the Turkish side of the border, Turkish jets were heard overhead.
At one point, sounds of jubilation were heard from Ras al-Ayn. One rebel shouted in Arabic: "The Syrian army fled. Did you see?"
The Syrian Observatory for Human Rights said warplanes carried out six airstrikes in al-Hasaka, including those at Ras al-Ayn.
Regime fighter jets also targeted the rebellious suburbs of Damascus on Wednesday, the Britain-based Observatory said. Heavy clashes between rebel units and Assad's troops were ongoing in the northern city of Aleppo, the Observatory said. The group relies on reports from activists on the ground.
Although the conflict has been grinding on for nearly 20 months, neither side has managed to strike a blow that could tip the balance.
Over the weekend, Syria's splintered rebel factions agreed to a U.S.-backed plan to unite under a new umbrella group that seeks a common voice and strategy against Assad's regime.
President Barack Obama said he's encouraged the opposition has formed a new, more representative leadership council, but the U.S. isn't ready to recognize the group as a "government in exile" or to arm it.
"We consider them a legitimate representative of the aspirations of the Syrian people," Obama said at a news conference at the White House.
France was the first Western country to formally recognize the newly formed opposition coalition as the sole legitimate representative of the Syrian people.
Obama said the U.S. wanted to make sure the group "is committed to a democratic Syria, an inclusive Syria, a moderate Syria." He also said the U.S. isn't considering sending weapons to the opposition because of concerns the arms might fall into the hands of extremists.
"We have seen extremist elements insinuate themselves into the opposition and one of the things that we have to be on guard about, particularly when we start talking about arming opposition figures, is that we are not indirectly putting arms in the hands of folks that would do Americans harms, or do Israeli harm or otherwise engage in actions that are detrimental to our national security," he said.
The outgunned rebel fighters want arms - including critical anti-aircraft batteries - from main regional backers such as the wealthy Gulf states and Turkey.
Syrian Information Minister Omran al-Zoebi brushed off the new opposition group as a "desperate attempt" to undermine Syrians' morale.
Foreign ministers from the main Gulf Arab bloc - which includes key rebel backers Saudi Arabia and Qatar - met Wednesday in the Saudi capital Riyadh to discuss the crisis, according to the official Saudi News Agency. The talks were expected to bring in visiting Russian Foreign Minister Sergey Lavrov, whose nation is an important ally of Syria.
Associated Press writers Elizabeth A. Kennedy in Beirut, Lauren E. Bohn in Jerusalem, Mehmet Guzel in Ceylanpinar, Turkey, and Matthew Lee in Washington contributed to this report.