Brush fires from stray mortar bombs were still ablaze on the Golan Heights on Friday as Israeli farmers returned to their fields, a day after battles in Syria's civil war reached a UN-manned border crossing.Once the smoke clears, Israel could find itself facing more trouble from multiple threats on its northern front.Hezbollah threatIsrael has struck inside Syria at least three times, according to foreign reports, in the past few months, each attack against what it believed to be weapons for Hezbollah, whose leader Hassan Nasrallah has threatened to open a new front against Israel on the Golan.One senior Israeli official briefed on intelligence said Nasrallah's words seemed to be backed by action."Hezbollah appears to be making inroads on the Syrian-held Golan too. This would seem consistent with what Nasrallah pledged. There aren't Hezbollah 'boots on the ground' there yet but the infrastructure is being built," said the official, who spoke on condition of anonymity.The official added that Hezbollah had much to gain from fighting on behalf of its longstanding patron Assad. Hezbollah, he said, was acting under assurances it would be rewarded by Assad in the form of arms transfers.Hezbollah may be bolstered by its joint victory alongside Assad's forces against rebels in the battle over recent weeks for the Syrian town of Qusair, watched closely in Israel."It is our understanding that Qusair was basically a Hezbollah operation, from the planning to the handling of key weapon systems," the official said. "Hezbollah crews were even operating Syrian T-55 and T-54 tanks there, as well as all significant artillery systems."But Hezbollah's involvement in Syria could also have a silver lining as far as Israel is concerned. Another Israeli official said Israeli intelligence assessed that up to 500 of the group's fighters have been killed in Syria.That estimate was higher than others and Hezbollah itself has not said how many of its men have died in Syria.Rabi said Hezbollah, which fought a war with Israel in 2006, was losing more than just men in its battles for Assad."Hezbollah is losing its legitimization and prestige. After the 2006 Lebanon war, Hezbollah was hailed in the Muslim and Arab world for carrying the torch in the fight against Israel. But with its entrance into Syria, it has made itself a target for Sunnis in Lebanon and in the entire world," Rabi said.On Thursday, Syrian President Bashar Assad's forces beat back rebels who seized the Quneitra crossing on the Golan, a strategic plateau captured by Israel in a 1967 Middle East war. The battles sent UN peacekeepers to their bunkers and prompted Austria to announce it was pulling its men out of the mission.Israel is now concerned the entire United Nations Disengagement Observer Force (UNDOF) is on the brink of unravelling - a scenario that could bring further escalation along what has been for decades a quiet frontier with Syria.The peacekeepers, in place under a 1974 disengagement agreement after Israel and Syria fought a second war on the Golan, had mostly found their biggest enemy to be boredom.But their quiet presence has been highly symbolic - an affirmation of a status quo under which the two countries, which last held peace talks 13 years ago, avoided direct conflict that could lead to all-out war."If there are no Austrians there is no UNDOF. They were the core force," an Israeli diplomatic source, speaking on condition of anonymity, said. "It will be very hard to find a replacement."Russian President Vladimir Putin, an Assad ally, said on Friday that he was willing to send troops to fill in for the Austrians. The UN subsequently announced that, as a permanent member of the UN Security Council, Russia could not send peacekeepers to the Golan, as per an agreement between Israel and Syria.On high alert over escalating fighting between Assad's forces and his enemies in the Syrian-controlled parts of the Golan, Israel has started in recent months to adjust its deployment along the front. Shelling and machine gun fire have occasionally spilled over into Israeli-held territory.The IDF has revived once-abandoned outposts on the Golan and sent up regular forces to take the place of reservists. Israeli leaders have spoken particularly of a future threat posed to peace on the Golan by jihadi fighters now battling against Assad's forces.According to foreign reports, Israel has launched air strikes on Syria to prevent weapons transfers to arch-enemy Hezbollah, an Iran-backed Lebanese militant group fighting on Assad's behalf.However, it has shown few other signs of preparing to intervene in the civil war and has avoided taking sides.Unlike his Western allies, Prime Minister Binyamin Netanyahu has stopped short of calling for an end to Assad's rule.Bad news for Assad is generally seen as good news for Israel, which views him as the center of a network of enemies linking Iran to Lebanon's Hezbollah and Hamas, the Islamist movement which controls the Gaza Strip."From a selfish Israeli point of view, what is happening in Syria is a huge positive development for Israel. This axis of radicalism is now broken," said Amos Yadlin, head of Israel's Institute for National Security Studies.But Israel also knows that its enemy's enemy is not necessarily a friend."A complete victory by either side would not be an optimal situation," said Uzi Rabi, head of the Moshe Dayan Centre for Middle East Studies. "The current situation is in a way optimal for Israel ... and it will most likely go on for months if not years."On the Golan on Friday, Israeli firefighters put out brush fires from Thursday's fighting. As gunfire from Syria echoed at times in the distance, Israeli and Druze farmers tended to their cherry orchards. Israelis peered through binoculars and watched shells on the Syrian side send up clouds of smoke.Along one road, two Israeli soldiers, one of them armed with an anti-tank missile, crouched on the ground, gazing in the direction of Syria.