Russian and Syrian jets stepped up their bombing of an Islamic State bastion along the Jordan-Israel border in southwestern Syria, as 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
A former resident in touch with relatives said thousands of civilians whose villages have been bombed have fled to the safety of areas either held by the army or rebels.
Another source familiar with the situation said Islamic State had actually been able to expand its territory over the last 20 hours by seizing at least 18 villages abandoned by other rebels under the banner of the Free Syrian Army (FSA).
Islamic State was taking advantage of the collapse of its ideological FSA rivals which it views as apostates, the source said.
The United States once armed the southern FSA rebels, but told them at the start of the Russian-Syrian offensive not to expect its intervention. While cutting other aid to the rebels, Washington had continued to provide those fighting Islamic State with weapons, the source added.
The Syrian army said its aerial strikes and shelling of militants in the Yarmouk Basin - the only territorial pocket held by the hardline Sunni fundamentalists in the country's southwest - had killed "tens of terrorists" in a campaign whose goal it said was to crush militants.
The army and its allies have been pushing to expand their foothold near the Golan frontier by negotiating surrender deals with rebel groups and allowing them to move to opposition-held areas in northern Syria.