Thousands of people have fled after the Syrian regime and Russian forces launched a fresh assault on rebel-held Daraa in south west Syria.
At least 50,000 people have been made “internally displaced” since the assault began last month.
Thousands are trapped at the borders of British allies Jordan and Israel.
An Israeli military statement said, “We will not allow passage of Syrians fleeing the country.”
Israel has also bolstered its military forces in the occupied Golan Heights, which is close to the fighting.
And airstrikes by a British-backed coalition—which is also fighting in Syria—continue.
The coalition last week admitted to killing 40 civilians in a strike on a school in March 2017.
The United Nations said 150 civilians were actually killed.