Syrian rebels control almost all the villages near the frontier with the Israel-held Golan Heights, an Israeli government minister has said.
The development takes the conflict dangerously close to the Jewish state and raises the possibility of an armed clash with the region's strongest power.
"Almost all of the villages, from the foot of this ridge to the very top, are already in the hands of the Syrian rebels," said Mr Barak, who was accompanied by Prime Minister Benjamin Netanyahu.
"The Syrian army is displaying ever-diminishing efficiency."
The civil war in Syria has renewed tensions over the Golan Heights, a strategic plateau which Israel captured from Syria in 1967.
Despite hostility between the two countries, Syria has been careful to keep the border quiet since the 1973 Middle East war.
But in recent days Israeli troops have fired into Syria twice after apparently stray mortar shells flew into Israel-held territory.
On Wednesday an Associated Press journalist said an Israeli helicopter was patrolling the border area and gunfire could be heard. The source of the gunfire was not immediately clear.
While it is widely believed that Mr Assad does not want to pick a fight with Israel, there are fears that the embattled Syrian leader may try to draw Israel into the fighting in a bout of desperation. Israeli officials believe it is only a matter of time before Syrian rebels topple the long-time leader.