Austria says it will pull out of a UN force on the Golan Heights after battles between Syrian troops and rebels there, in a blow to a mission that has kept the Israeli-Syrian war front quiet for 40 years.
Israel, anxious for the international mission to remain in place, worried that the Golan could become a springboard for attacks on Israelis by Islamist militants fighting to oust Syrian President Bashar al-Assad.
"While appreciating Austria's longtime contribution and commitment to peacekeeping in the Middle East, we nevertheless regret this decision and hope that it will not be conducive to further escalation in the region," the Israeli Foreign Ministry said in a statement.
But the departure of the Austrians, who make up about 380 of the 1000-member United Nations Disengagement Observer Force (UNDOF), threatens the whole operation.
"Austria has been a backbone of the mission, and their withdrawal will impact the mission's operational capacity," said UN spokeswoman Josephine Guerrero.
The UN Security Council will meet on Friday (local time) to discuss the Austrian withdrawal. British Ambassador Mark Lyall Grant, president of the council this month, said peacekeeping officials were meeting with contributing countries to see whether any states would be willing to offer troops to replace the Austrians.
"We consider UNDOF to be an extremely important mission," Lyall Grant said. "We support it and we want it to continue."
Anti-Assad rebels briefly seized the crossing between Israel and Syria, sending UN staff scurrying to their shelters before Syrian soldiers managed to push them back and reassert their control of Quneitra.
The rebel attack appeared to be an attempt to regain some momentum after Assad's forces, backed by well-trained Lebanese Hezbollah guerrillas, on Wednesday seized control of Qusair, a town on a vital supply route close to Lebanon.
Meanwhile, Russia announced it has deployed a naval unit to the Mediterranean Sea in a move President Vladimir Putin said was to defend Russian security, as Moscow faces off with the West over its support for Assad's government.
"This is a strategically important region and we have tasks to carry out there to provide for the national security of the Russian Federation," Putin said.
Syrian government troops and their allies have won a string of successes in recent weeks, boosting Assad at a time when the United States and Russia are struggling to organise a peace conference aimed at ending the civil war that has killed more than 80,000 people.
Looking to ram home their victory, Assad's troops have turned their fire on villages northeast of Qusair, where hundreds of rebels and civilians were holed up, prompting one group of activists to issue a desperate plea for rebel support.
"God has given us the strength to persevere, but until when only God knows. We beg you to move as quickly as possible to rescue us," said a message posted on social networking sites.
Shortly afterwards, Syrian television announced that the army had "restored security and stability" to one of the villages in its sights - Debaa.
France, which earlier this week accused Assad of deploying nerve gas in the civil war, said on Wednesday the situation on the ground needed to be "rebalanced" after the fall of Qusair, but did not say how that could be achieved.
Russia said it was concerned that allegations of gas attacks might be used as a pretext for foreign intervention.
"I do not rule out that somebody wants to use it to state that a red line has been crossed and a foreign intervention is necessary," Russian Foreign Minister Sergei Lavrov told a news conference in Moscow with his German and Finnish counterparts.
Western countries have shown little appetite for getting sucked into the Syrian conflict, but there is also a clear aversion to letting Assad, heavily backed by Shi'ite Iran and their Hezbollah associates, emerge victorious.
France and Britain last month pushed the European Union to drop its ban on arming the rebels, who are mainly Sunni Muslims. London and Paris have not yet said if they plan to arm the fighters. They wanted the ban lifted to apply pressure on Assad.
The International Committee of the Red Cross said it was negotiating with Syria to reach areas surrounding Qusair to deliver medical assistance to the wounded. Humanitarian groups have estimated that up to 1500 people might need help.
"Today the conflict is extremely fragmented, and this is one of the biggest operational challenges for the ICRC," said Robert Mardini, the head of Red Cross operations in the Middle East.
Qusair lies along a corridor through the central province of Homs, linking the capital Damascus to the coastal heartland of Assad's minority Alawite sect, an offshoot of Shi'ite Islam.
Many rebels and civilians fled the town early on Wednesday, heading to the villages of Debaa, 5 kilometres (3 miles) northeast, and Buwayda, another 7 km in the same direction.
"We have a large number of civilians and wounded in Buwayda," said activist Mohammed al-Qusair.
Russia, which has thrown its weight firmly behind Assad West, cautioned Damascus that the conflict could only be resolved through diplomacy.
"The undoubted military success of the government forces should not in our opinion be used by anyone to create the illusion about the possibility of solving all the problems faced by Syria by force," Russia's Foreign Ministry said in a statement.
With sectarian divisions widening in the region, the leader of Sunni Islamist group al Qaeda, Ayman al-Zawahri, urged Syrians to unite against Assad and thwart what he called US plans to set up a client state to safeguard Israel's security.
The longer the civil war has continued, the more neighbouring countries have felt the spillover.
Two men died after a gunfight with Lebanese soldiers near the Syrian border early on Thursday, while the Turkish military said one Turkish soldier was wounded in a clash with gunmen who were part of a group of about 500 people trying to reach Turkey.
Israel, which has kept a wary eye on the Golan Heights, exchanging sporadic fire with assailants and warning of swift retaliation should its forces come under attack, said it expected the United Nations to maintain the monitoring mission.
Austria defended its decision to leave, saying it could no longer justify its troop presence.
"Freedom of movement in the area de facto no longer exists. The uncontrolled and immediate danger to Austrian soldiers has risen to an unacceptable level," Austrian Chancellor Werner Faymann and his deputy Michael Spindelegger said in a statement.
Japanese and Croatian troops also have left the UNDOF in recent months, while the Philippines has said it might leave after Syrian rebels held its peacekeepers captive.