Syrian forces loyal to President Bashar al Assad fought off an attempt by rebels to seize the sole crossing between Syria and Israel yesterday, while army troops sought to ram home strategic gains further to the north.

A day after losing control of Qusair, an important town close to the Lebanese border, rebels tried to grab back the initiative with an assault on Quneitra – a demilitarised zone on the Golan Heights that is patrolled by the United Nations.

For the first time since the start of the uprising in March 2011, the rebels briefly took control of the area, sending UN peacekeepers scurrying to their bunkers. Israeli sources said Syrian forces wrested back the site after fierce fighting.

Austria said it would withdraw its peacekeepers from the UN monitoring force because of the fighting.

Austrians account for about 380 of the 1000-strong UN force monitoring a ceasefire between Syria and Israel, and their departure will deal a serious blow to the mission.

"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 joint statement.

"This morning's developments show that a further delay (in pulling out soldiers) is no longer justifiable," the statement said.

Austria's defence ministry was in contact with the United Nations' department of peacekeeping operations "to create the conditions for an orderly withdrawal of Austrian peacekeepers", it added.

Pro-government troops have won a string of successes in recent weeks, boosting Mr Assad at a time when the United States and Russia are struggling to organise a peace conference aimed at ending the civil war, which has killed more than 80,000.

Activists pushed out of Qusair this week issued a desperate plea for help, saying they were cornered by both Syrian troops and their powerful Lebanese allies.