In this photo taken on Tuesday, Sept. 16, 2014, U.N. peacekeepers observe Syria's Quneitra province at an observation point on Mt. Bental in the Israeli-controlled Golan Heights, overlooking the border with Syria. (Tsafrir Abayov/AP)

Israel has some new neighbors. Al-Qaeda-linked rebels have, for the first time, established a permanent presence on the Syrian frontier.

For 40 years, Israel and Syria have kept the peace along the 1974 armistice line here, a cease-fire overseen by U.N. troops from countries such as Fiji and the Philippines, who patrolled a demilitarized buffer zone populated by apple farmers.

Today, the U.N. peacekeepers are mostly gone. And so is the Syrian government.

In a series of sharp, decisive skirmishes over the past few weeks, Syrian army forces loyal to President Bashar al-Assad were swept aside by al-Qaeda-linked rebels from Jabhat al-Nusra and other anti-Assad militants along nearly the full length of the Golan buffer zone.

In August, Syrian rebel forces flushed U.N. peacekeepers from their base near the Quneitra border crossing and kidnapped a contingent of 45 Fijian soldiers stationed nearby. The Fijians were released unharmed after two weeks, but the entire contingent from Fiji has since packed up and left.

In this photo taken on Tuesday, Sept. 16, 2014, U.N. peacekeepers observe Syria's Quneitra province at an observation point on Mt. Bental in the Israeli-controlled Golan Heights, overlooking the border with Syria. (Tsafrir Abayov/AP)

A generation of Israeli commanders has warily eyed the Syrian side. But things have gotten demonstrably worse in the past month. With U.N. peacekeepers absent, the Syrian army routed and al-Qaeda affiliates in the Jabhat al-Nusra in control, the sleepy borderland is more unstable than at any time in the past four decades.

“Who is in control over there? We don’t know,” said Col. Nir Ben David, a senior officer with the Northern Command of the Israeli Defense Forces.

He pointed to scattered farm towns in Syria.

“Those aren’t just villages now. Those are rebel bases ,” Ben David said.

For now, Israeli army leaders say their intelligence leads them to believe that the Islamist militants in the Syrian opposition are most eager to topple Assad, not tackle Israel. But they wonder for how long.

“I think it is only a matter of time before they turn to Israel, a year, half a year. All other radical groups in the region — in the Sinai, in Lebanon — at some point they stop fighting their local enemies and start carrying out attacks against Israel,” said Eyal Zisser, an expert on Syrian affairs at Tel Aviv University.

Israel occupied the Golan Heights after the 1967 Arab-

Israeli war. During the 1973 Yom Kippur war, Syrian forces punched into the Golan but were pushed back by Israeli counter­attacks. Israel and Syria signed a cease-fire in 1974, which the United Nations monitors.

Syrian opposition fighters have been operating along the Golan frontier during most of the three-year civil war in their country, but in recent months they have consolidated forces and notched their belts with a string of victories. The rebel groups here — led by the al-Qaeda affiliates — control 90 percent of the boundary separating Syria and Israeli-controlled Golan, according to Israeli military commanders.

The majority of the 1,200 troops with the U.N. Disengagement Observer Force, known as UNDOF, have abandoned their bases in the demilitarized zone and are encamped on the Israeli-controlled side of the border. Last week, the Philippines announced that it was bringing its 300 troops home early.

Syrian rebels occupy the vacated UNDOF bases and observation posts in the buffer zone and control the single crossing gate between Israel and Syria, according to Israeli military officials.

“Under the 1974 armistice agreement, the Syrian army were not allowed to be close to the border with Israel,” said Ben David, who toured the border with journalists last week. “They were stationed quite far away. Their tanks were only allowed within [about nine miles] of the border. But now, the rebels have taken tanks and guns and they are sitting right on the border.”

In a statement, the United Nations said UNDOF is fulfilling its mandate, “continuously adjusting its activities and posture in response to the evolving security situation on the ground, which has continued to deteriorate.” The U.N. Security Council asserted on Friday that the force will not be disbanded.

The Syrian villages sit just meters from Israel’s high-tech “smart fence ,” bristling with cameras and sensors, constructed a year ago. Many of the larger buildings in the villages are stained black from smoke and fires, while others are destroyed — some from recent fighting, some from battles four decades ago. Jeeps and black SUVs can be seen moving along the broken roads. Rockets and artillery fire boom in the background.

Mortar, tank and artillery fire from the fight in Syria has reached Israel several times, injuring Israeli soldiers who patrol the border road and killing an Israeli teenager who was traveling there with his father this summer.

“From time to time, the Syrian army approaches and fires toward [the rebels], and the rebels fire back. I know that no party is interested in attacking Israel right now. But if the artillery goes over the border fence, that is a red line for us, and we have to respond,” Ben David said.

“This is the first time since 1974 that on the other side of the border in the Golan Heights we do not have the Syrian armed forces or the UNDOF forces in between. Now, there is a completely different situation,” said Shaul Shay, former deputy head of the Israel National Security Council.

“If my assumption is correct, that sooner or later they will turn their weapons against Israel, what is the best thing to do?” Shay said. “A preemptive operation to push them from the border? Or just wait and see? So far, Israeli policy is to wait and see. If they push back Nusra, then another group that’s even worse could fill the space.”

Booth reported from Jerusalem.