Russian and Syrian jets stepped up their bombing of an Islamic State (IS) bastion along the Jordan-Israel border in southwestern Syria, reports Agencies.
The militants pushed into areas abandoned by other rebel groups, diplomatic and opposition sources said.
Islamic State-affiliated forces entrenched in the Yarmouk Basin, which borders the Israeli-occupied Golan Heights and Jordan, also repelled a ground attack by the Syrian army and its allies, the sources added.
The agricultural area has become the main battleground in the sensitive border region after a major Russian-backed Syrian army offensive routed other rebel groups who were once backed by Washington, Jordan and Gulf states.
An intelligence source told Reuters 1,000-1,500 Islamic State fighters had been holding their ground despite the 10-day-old bombing campaign that he said had hit villages and caused "untold number" of civilian casualties
Meanwhile, Israel says it has carried out an evacuation of members of Syria's White Helmets civil defence group from a war zone in south-western Syria.
The Israel Defence Forces (IDF) said they had acted on a request from the US and European nations.
Some 422 volunteers and family members were taken to Jordan via the Israeli-occupied Golan Heights overnight.
The White Helmets describe themselves as a volunteer workforce that acts to save people in Syria's war zones.
Jordan said on Sunday 800 people including members of Syria's "White Helmets" civil defence group had been evacuated from southwest Syria where Damascus' forces are advancing against rebels, with Israeli officials saying they facilitated the mission.