Austria announced it is withdrawing 377 U.N. peacekeepers from the Golan Heights after Syrian rebels briefly overran a crossing point near the border with Israel on Thursday, a development that has deepened concerns the civil war is spreading to neighboring countries.

Fighting between President Bashar Assad's forces and mainly Sunni rebels has already spilled over Syria's borders into Turkey and Lebanon, where factions that support opposing sides have frequently clashed. From the Israeli side of the Golan, tanks and armored vehicles could be seen from about a half mile away in the Syrian-controlled part of the territory. Thick smoke and flames rose from the area and a large fire raged.

Also Thursday, Al Qaeda leader Ayman al-Zawahri tapped into the deepening Sunni-Shiite rift stemming from the conflict, calling on Sunnis everywhere to devote their lives, money and expertise to the overthrow the regime, set up Islamic rule in Syria and prevent a U.S.-allied government from taking over after Assad, whose regime is dominated by the Alawite sect, an offshoot group of Shiite Islam.

Al-Zawahri also urged Sunnis to "rise above their differences" and fight expanding Shiite influence in Syria. The authenticity of al-Zawahri's message, which came in an audio recording on the Internet, could not be independently confirmed but it was posted on a militant website commonly used by Al Qaeda.

Al-Zawahri has repeatedly called for holy war in Syria and has blasted Lebanese Shiite Hezbollah militants and Iran for supporting Assad. He joins other Sunni leaders who have used the sectarian undertones of the conflict to widen support for the rebels.

Hezbollah has a vested interest in the survival of Assad's regime. It deepened its involvement in the civil war by sending fighters over the past month to battle rebels in Qusair, an overwhelmingly Sunni town and an opposition stronghold in western Syria.

After a grueling three-week battle, the Syrian army on Wednesday regained control of Qusair, which has served as a conduit for shipments of rebel weapons, fighters and supplies smuggled from Lebanon. For the regime, the town was equally important since it lies between Damascus, the seat of Assad's government, and the Alawite heartland near the Mediterranean coast.

Islamic militant groups, such as the Al Qaeda-linked Jabhat al-Nusra, are the most organized and most effective force fighting on the rebel side in Syria. The U.S. and its European allies, who have backed the opposition, share Israel's concern about the influence of Islamic radical factions in the rebel ranks.

Highlighting the growing tension in the region, Jordan's Foreign Minister Nasser Judeh warned the Syrian ambassador there to stop criticizing his host country or risk expulsion. Syrian ambassador Bahjat Suleiman is an ardent critic of Jordan's policies toward his country, accusing it of siding with anti-Assad forces.

Austrian Chancellor Werner Fayman and Foreign Minister Michael Spindelegger said the fighting over the border position in the Golan, a demilitarized area in the plateau captured by Israel from Syria in 1967, made it necessary to withdraw their troops.

"The development ... has shown that further waiting can no longer be justified," a joint statement said.

The Israeli Foreign Ministry said it regretted the decision and hoped that it would not lead to "further escalation in the region."

Israeli officials say the fall of the fall of the "symbolic" Quneitra border position, which lies along an important route to Damascus, would have a greater impact on the Syrian war than the Jewish state. Quneitra is the only crossing along the cease-fire line and primarily serves the U.N. peacekeepers and Druse villagers moving between the Israeli- and Syrian-controlled Golan.

Israeli military officials, who earlier had said the crossing had fallen to the rebels, said the situation at the border by early afternoon remained "fluid" and fighting was still raging on the Syrian side of the border. The officials spoke on condition of anonymity because they were discussing a confidential strategic assessment.

The Britain-based activist group the Syrian Observatory for Human Rights said the rebels attacked Syrian regime checkpoints and their armored vehicles, and briefly took control of the crossing, located near the old city of Quneitra.

The activist group, which relies on a network on informants inside Syria, said there were intense clashes between regime forces and rebels in the area. No casualties were immediately reported.

Austria has contributed peacekeepers since the start of the Golan mission in 1974 to separate Israeli and Syrian forces, and its soldiers make up the largest contingent in the 900-strong force, which also includes peacekeepers from the Philippines and India. Croatia withdrew its contingent in March amid fears they would be targeted in fighting at the border area.

Israel and Syria agreed to creation of the U.N. Disengagement Observer Force after Syria launched military action in 1973, in a failed effort to retake the area it lost to Israel in 1967. The Israeli Foreign Ministry said it expected the United Nations to uphold that commitment.

The Philippine military announced that one of the country's peacekeepers was wounded in the leg by artillery or a mortar shell that landed at Camp Ziouni, a logistics base, earlier Thursday during fighting between Syrian government and rebel forces.

Syrian state-owned Al-Ikhbariyah TV denied that the rebels control the crossing, saying the army was pursuing "terrorists" — a term the government uses for opposition fighters — in the Golan.

The Israeli military did not confirm the crossing was overrun by Syrian rebels, but said the area had been declared a "closed military zone" and was off-limits to journalists because of fighting nearby.

The Syrian conflict started in March 2011 as peaceful protests against Assad's regime, then turned into a civil war after some opposition supporters took up arms to fight a harsh government crackdown on dissent.

The conflict has also created a refugee flow of some 1.6 million people, most of whom are currently in neighboring countries.

The aid group Doctors without Borders warned Thursday that the needs of civilians in Syria and neighboring countries far outstripped the relief currently being supplied, urging the international community ahead of a humanitarian conference in Geneva on Friday to ramp up its aid supply .

There have been previous incidents in the Golan, with gunfire and mortar shells striking the Israel-controlled zone in recent months. Israel believes most of the fire is incidental spillover but in some cases it has said the strikes were deliberate.

In one such incident last month, Syrian troops targeted an Israeli jeep they said had crossed the cease-fire line into the Syria-controlled sector. Syria said it launched two missiles in self-defense, accusing Israel of violating the cease-fire deal.

In the latest spillover in Lebanon, 10 rockets fired from Syria hit the eastern Lebanese city of Baalbek shortly before midnight Wednesday, wounding one person and causing damage, according to a Lebanese army statement.

The statement said troops clashed with gunmen near the border town of Arsal after gunmen fired at a military post in the area. It said two gunmen, one of them Syrian, were killed in the overnight fighting and the rest fled.