Thousands of civilians have fled the the southern opposition stronghold that borders Jordan and the Israeli-occupied Golan Heights as the Syrian regime intensified its offensive to retake the area.
The US warning comes following Syrian regime's attack on a de-escalation zone in northeastern Syria held by US-backed Syrian Democratic Forces.
The Syrian regime says it has retaken the last neighbourhoods in southern Damascus held by Daesh and declared the capital and its surroundings "completely safe" from militants for the first time in nearly seven years.
Syrian regime media accused Israel of launching missiles against its forces near Damascus.
An advanced Russian multi-role fighter plane crashed in Syria on Thursday after take-off, killing both members of its crew, the Russian military was cited as saying by Russian news agencies.
Most of the dead were Iranians fighting for the Assad regime, the Syrian Observatory for Human Rights said. The attack targeted an arms depot for surface-to-surface missiles at a base in northern Syria known as Brigade 47.
Syrian Observatory for Human Rights says the overnight raids targeted Yarmouk Palestinian refugee camp and a small enclave south of the capital Damascus under the control of militants.
The United States, Britain and France launched strikes against the Syrian regime in response to an alleged chemical weapons attack after mulling military action for nearly a week.
Russian defence ministry spokesman Igor Konashenkov says Britain told the White Helmets, who act as first responders in opposition-held areas, to fake the suspected chemical attack in the town of Douma in which at least 42 people were killed.
The US wants a UN Security Council vote Tuesday on a US draft resolution to establish a new inquiry into who is to blame for using chemical weapons in Syria, diplomats said. The move comes after a suspected chemical attack on Douma killed dozens.
Air strikes killed at least 27 civilians on Friday in the last opposition-held town in Syria's eastern Ghouta, a monitor said, the first bombardment there since talks sputtered over a rebel withdrawal.
The first batch of fighters from Jaish al Islam are being transported to Jarablus from the town of Douma as part of an evacuation deal, leaving behind only undecided hardline fighters, regime media and a war monitor say.
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