At least 15,000 Syrian refugees fleeing fighting in northern Aleppo province have gathered at a border crossing with Turkey, UN and Turkish officials said.
The frontier is shut but Turkey has said it is prepared to feed and shelter the refugees.
In the past few days, the Syrian army backed by Russian air power has made a series of gains in Aleppo province.
The advance threatens to surround the city of Aleppo, Syria's largest, landing a major blow to the rebels.
Nato has accused Russia of "undermining" Syrian peace efforts through its strikes, which it says are mainly aimed at opposition groups.
But Russia insists it only targets what it calls terrorists.
Sami Kekhia, a Syrian activist, told Al Jazeera that "People are fleeing the deadly Russian air strikes and the Syrian government offensive. Russian air strikes have not stopped: when a Russian warplane targets a building that building turns to dust.
As Seneca once said, “What need is there to weep over parts of life? The whole of it calls for tears.”