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Heavy fighting broke out Thursday in the Golan Heights, as Syrian rebels and troops loyal to President Bashar al-Assad battled to take and hold a U.N.-manned border crossing linking Israel and Syria.

During the hours-long clash, Syrian rebels briefly took control of the Quneitra crossing but abandoned it later to Syrian army troops. The two sides also battled in the city of Quneitra, about a mile away.

The fighting, which Israeli military officials described as “intense,” forced U.N. peacekeepers to flee the area and return to their fortified camps.

Saying the Golan Heights had become too dangerous, Austria announced Thursday that it is pulling all its soldiers out of the buffer zone where members of the U.N. Disengagement Observer Force have served since 1974, helping to monitor a cease-fire between Syria and Israel.

“The uncontrolled and immediate danger to Austrian soldiers has risen to an unacceptable level,” the Austrian chancellor said in a statement. Israel’s Foreign Ministry responded by demanding that the United Nations find another contingent of troops to replace the Austrians.

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The departing Austrians comprise about one-third of the 900 troops who patrol the 46-mile-long demilitarized zone, along with contingents from the Philippines and India. Israel captured the Golan Heights from Syria during the 1967 Arab-Israeli war.

All the fighting Thursday took place on the Syrian side of the boundary, although two mortar shells landed in the Israeli-controlled Golan Heights, according to Israeli military officials who spoke on the condition of anonymity because reports from the battle were preliminary.

From the hilltops on the Israeli side, large clouds of black smoke rose were visible amid the sounds of mortars, grenades and sustained automatic-weapon fire as Syrian army troops and rebels fought in and around Quneitra, strategically located on the road to Syria’s capital, Damascus.

The military officials said the mortar fire into Israel appeared accidental. Israel evacuated two Syrians who were injured in the fighting and transported them to area hospitals.

The Quneitra crossing is the only one between the neighboring enemy states, and it is opened only to allow U.N. peacekeepers and local Druze inhabitants to pass. The Druze population is split between Syria and Israel, and its members use the crossing to bring their apple harvests into Syria and to bring families together to celebrate weddings.

The battle at the crossing, within sight of Israeli troops, is likely to heighten Israel’s fears that the two-year-old civil war raging in Syria could spill over its borders.

The Golan Heights boundary was for almost 40 years one of Israel’s most peaceful. But in recent months, Filipino peacekeepers have been kidnapped by Syrian rebels; Syrian army troops fired machine guns at an Israeli patrol before drawing retaliatory rocket fire from Israel; stray rocket and mortar rounds have repeatedly landed in Israeli-occupied farms in the Golan; and Israel launched three airstrikes against targets in Syria where sophisticated weapons were allegedly being transferred to the Lebanon-based political and military movement Hezbollah, considered a terrorist organization by Israel adn the United States.

Thursday’s attack on the crossing came a day after rebels retreated from the key Syrian town of Qusair following a two-week battle against the Syrian army and Hezbollah militants. Military analysts suggested that it may have come in reaction to that blistering defeat.

“It’s hard to see anything strategic in terms of value, given that the crossing leads into Israeli territory,” said Charles Lister, an analyst at IHS Jane’s Terrorism and Insurgency Center. “It’s feasible that this is an attempt to deflect attention from their defeat in Qusair, as they know there will be a lot of international attention.”

Aviv Oreg, a former head of the al-Qaeda and global jihad desk in the Israel Defense Forces military intelligence directorate, concurred that the attack was more symbolic than strategic. “The rebels are saying: ‘We’re still here. We exist. We lost a battle in Qusair, but we haven’t lost the war,’ ” he said.

Concerns remain about the fate of thousands of civilians who remain trapped in and around Qusair, despite reports that a safe exit to Lebanon had been secured by Hezbollah and the Syrian army to encourage civilians and fighters to leave.

Abu Rami, an activist based in Homs province, where Qusair is located, said most had only been able to get to the nearby village of Bouaida. More than 1,000 civilians and fighters, many in dire need of medical attention, were now trapped in the village with little access to medical care, he said.

Israeli officials worry that forces loyal to Assad could try to provoke a response from Israel or that the ungoverned areas in the Golan Heights could serve as staging grounds for attacks against Israel by Hezbollah or other Islamist militants.

“They want to provoke a response by Israel,” said Jacques Neriah, former deputy head for assessment of the Israeli military intelligence branch. “They want us to shoot back.”

Morris reported from Beirut.