Rebel factions in Syria, including an al Qaeda affiliate, captured a border crossing between Syria and Israel, Israel's military and Syrian opposition activists said, the second major loss of a strategic site to extremist groups within days.
Syrian forces were still fighting to recapture the crossing late Wednesday around Quneitra, located in the disputed Golan Heights area between both countries, 45 miles southwest of the Syrian capital Damascus. There was no evidence the rebels were attempting to infiltrate Israel.
The move followed the capture of an air base in northern Syria on Sunday by the group known as Islamic State.
The developments raised questions over whether President Bashar al-Assad's regime is so weakened by Syria's more than three-year civil war that it cannot defend strategic areas.
Some observers suspect the regime is intentionally leaving the sites to militants to compel foreign powers to abandon their quest to topple Mr. Assad and instead cooperate with him to battle a common enemy in the form of jihadists.
The loss of the Quneitra crossing came as Mr. Assad announced a government shuffle that kept the prime minister and most senior ministers in place, including Defense Minister Maj. Gen. Fahd al-Freij. Some regime loyalists had called this week for Gen. Freij's ouster, blaming him for a string of recent defeats at the hands of militants and the high casualties suffered by the Syrian Army.
Syrian opposition activists said rebels launched the offensive on the Quneitra crossing early on Wednesday and that at least eight factions took part, including units of the Western-backed Free Syrian Army, the al Qaeda-linked Nusra Front and other allied Islamist groups.
In addition to Syrians, fighters include Jordanians, Palestinians residing in Syria and some other foreigners, the activists said.
Several video clips posted on YouTube by some of these rebel groups shows an area around a border fence and observation tower engulfed in heavy white smoke amid the sound of explosions and crackle of gunfire.
Rebels said they were in control of the crossing and what Syrians call "the ruins of Quneitra," the remains of a town destroyed during the 1973 Arab-Israeli war.
Neither the Syrian government nor official media outlets commented on the development. Damascus-based radio station Sham FM quoted unnamed officials confirming the rebel assault on the crossing but said battles were ongoing.
"The Syrian side of the crossing was overrun today by opposition forces—several different groups dominated by Jabhat al Nusra," said Lt. Col. Peter Lerner, a spokesman for the Israel Defense Forces, using the name of the Nusra Front in Arabic.
Israel's military declared the border near Quneitra a closed military zone after one officer received a bullet wound and three mortars landed in Israeli territory. It said Israel returned fire toward Syrian government positions to deter hostilities from spilling over.
Col. Lerner said Syrian rebels took control of the crossing for several hours one year ago but were later repelled by the Syrian Army.
He said that while Israel considers the presence of the militants at the border crossing to be a threat, Israel has been preparing for the destabilization of the frontier for years by bolstering its border fences, adding surveillance equipment and deploying elite forces to the border.
He said the Nusra Front and other Islamists already control swaths of territory along the cease-fire lines between Israel and Syria on the Golan Heights. He said Israel's deployment in the Golan will continue to be defensive.
"The security establishment identified this trend a long time ago," said Amos Gilad, a senior official in Israel's Defense Ministry in an interview with Israel Radio. "The basic deployment has prevailed for a long time."
The demilitarized zone in the Golan Heights was established in the months after the 1973 Arab-Israeli war. The zone, as well as the crossing, is monitored by United Nations peacekeepers. For years it was one of the quietest Arab-Israeli borders in the region.
There is very little traffic at the border because Israel and Syria are technically in a state of war. Only Druse from the Golan Heights are allowed to use the crossing for religious pilgrimages, university study in Syria, and for marriages.
After the start of the uprising against Mr. Assad in March 2011 the area around Quneitra and the demilitarized zone became a haven for defectors from the Syrian army. Fighting between regime forces and rebels in the area started in mid-2012 and gradually escalated until rebels succeeded in April 2014 in capturing several strategic hills in the area.
Opposition activists say a large influx of Islamists have invaded the area around Quneitra this year, including Nusra Front fighters, because Jordan prohibits them from operating in neighboring Deraa province.
Jordan, which shares a border with Israel and Syria, has tried to tread lightly in the Syrian conflict, allowing weapons, food and humanitarian aid to flow to rebels in Deraa, even hosting covert rebel training on its territory. But it hasn't totally severed ties with Damascus.
More Islamists flocked to the area in July after the Islamic State ousted the Nusra Front and other rivals from the eastern province of Deir Ezzour, activists say.
In a news conference in Damascus on Sunday, Syrian Foreign Minister Walid Moallem said if the West wanted to roll back the threat of militants in Syria it had to stop backing all rebels fighting the regime and cooperate with it instead but that this had to be done urgently before it's too late.
"I call upon everyone to feel the danger and to take the initiative to cooperate [with us] for the sake of their national interest," he said.