The rebel effort to overrun the Quneitra region along the cease-fire line separating Syria and Israel has heightened worries that Islamic extremists among those fighting President Bashar Assad could take over the front line with Israeli troops and gain a potential staging ground for attacks on the Jewish state.
The frontier has largely been calm in the nearly four decades since the two countries fought a war over the Golan Heights that ended with a U.N.-monitored cease-fire. But Israeli military officials have expressed concern that a rebel takeover could upset the calm maintained by Assad and his predecessor and father the late Hafez Assad.
Those fears have been compounded by increasing influence wielded by extremist groups over the divided rebels and the increasing international isolation of the regime.
"We are seeing terror organizations gaining footholds increasingly in the territory," said Lt. Gen. Benny Gantz, Israel's military chief at a conference in Israel last week. "For now, they are fighting Assad. Guess what? We're next in line."
One of the worst-case scenarios as Syria enters its third year of conflict is that neighboring countries such as Israel or Lebanon could be drawn in.
Israel says it is trying to stay out of Syria's civil war, but it retaliated for sporadic Syrian fire that spilled into Israeli communities on the Golan Heights on several occasions over the past few months.
When Israel bombed targets inside Syria said to include a weapons convoy headed for Hezbollah in Lebanon in January, Syrian opposition fighters derided Assad for not retaliating.
The Syrian rebels are made up of dozens of groups including the powerful, al-Qaida-linked Jabhat al-Nusra, which has been labeled a terrorist organization by the Obama administration. One of the groups involved in the Golan fighting, the Yarmouk Martyrs Brigade, is an Islamic militant group.
The rebels have largely been beaten back since they seized control of at least one Druze village and parts of several others in Quneitra province near the 1974 disengagement line.
The Britain-based Syrian Observatory for Human Rights said it had documented the deaths of 35 opposition fighters and contact had been lost with more than 20 others believed to have died in the fighting Wednesday and Thursday.
At least six pro-Assad fighters were also killed and dozens of others were wounded, according to the activist group.
For now, opposition fighters in Syria appear to be focused on toppling Assad, and there is little to suggest that they may be planning to turn their guns on Israel.
Rami Abdul-Rahman, director of the Observatory, said fighters were rallied around the same goal of ousting Assad. "But when there's chaos, who knows what might happen," he said.
Anthony Skinner, Middle East-North Africa chief at the British risk analysis firm Maplecroft, said the number of armed groups fighting in Syria makes it difficult to predict whether militant Islamists would contemplate an attack on Israel if the opportunity arose.
"Pragmatists would not want to provoke a riposte from Israel that could potentially play into Bashar Assad's hands," he said.
Skinner and other analysts said the fall of Quneitra on its own would be unlikely to trigger Israeli intervention, which would increase the risk of a war with the Syrian-backed Lebanese Hezbollah militant group.
"The worst thing Israel could do is get involved. Right now there is no immediate danger for Israel and it shouldn't get involved," said Eyal Zisser, professor of Middle Eastern studies at Tel Aviv University.
The Golan front has been mostly quiet since 1974, a year after Syria and Israel fought a war during which Damascus tried to retake the plateau, and briefly captured Quneitra, before the two sides agreed to disengage from the area.
The United Nations Disengagement Observer Force, known as UNDOF, was established to monitor the cease-fire in May 1974 by a U.N. Security Council resolution.
An Israeli military official this week said the entire border area has become a "playground" for skirmishes between rebels and the Syrian army.
Last month, members of the Yarmouk Martyrs Brigade held 21 Filipino peacekeepers hostage for four days, raising concerns about the future of U.N. operations in the area.
Syrian rebels in recent weeks have been making inroads in towns and villages in the frontier area stretching from the border with Jordan in the south to suburbs southwest of the capital, Damascus.
On Saturday, they seized control of a major air defense base used by the 38th Division after a 16-day siege in the south near the Jordanian border, killing its commander, activists said. The Yarmouk Martyrs Brigade also announced in a statement on its website that the base had been seized.
They also seized several army checkpoints, clearing a 25-kilometer (15-mile) stretch along the Syrian-Jordanian border, according to the Observatory.
Islamic extremists have risen in the mostly Sunni rebel ranks, thanks in large part to their superior organization and recruiting skills. That has led to a rise in suicide attacks targeting Assad's regime, including a bombing Thursday that killed 50 people, including one of Syria's best-known pro-government Sunni clerics, inside a Damascus mosque.
Mourners lifted the coffins of Sheik Mohammad Said Ramadan al-Buti and his grandson who also was killed, draped in white cloth, on their shoulders amid shouts of "God is Great" during the funeral procession on Saturday in Damascus.
Security forces sealed off all roads leading to the landmark eighth century Omayyad Mosque for the funeral of the 84-year-old cleric, who was the mosque's imam, was held.
His assassination was a blow to Assad, who vowed Friday to avenge his death, saying he would "purge" the country of the militants behind the attack in the heart of the capital.
Mourners carried al-Buti and his grandson's coffins, draped in white cloth, on their shoulders amid shouts of "God is Great."
Church bells tolled and mosque minarets in the ancient city blared "God is Great" during the funeral procession. State TV said al-Buti was buried in a courtyard at the rear of the mosque near the tomb of Saladin, a medieval Muslim warrior.
The choice enraged many among the opposition who considered it an affront to Syria's history and cultural heritage.
In Egypt, members of Assad's own minority sect who are opposed to his regime began a two-day meeting that organizers described as the first of its kind to discuss concerns about their fate in a post-Assad Syria.
Rebels fighting to end Assad's rule are mostly from the country's majority Sunni sect. Assad is Alawite, a Shiite offshoot of Islam.
Members of the Alawite community who make up about 12 percent of Syria's population have either rallied behind Assad or stayed quietly on the sidelines of the civil war.
The meeting of about 50 Alawites reflects fear the minority would fall victim to revenge killings and assassinations should Assad's regime fall.
They plan on seeking assurances from opposition chief Mouaz al-Khatib who may attend the meeting on Sunday.
Associated Press Writer Albert Aji in Damascus, and Josef Federman and Ian Deitch in Jerusalem contributed to this report.