Tens of thousands of Syrians sought refuge in Turkey on Saturday, while those who have already made the crossing waited and worried about the fate of their families on the other side of the border.

The border gate at Oncupinar, near the southern city of Kilis, was closed, but that did not stop dozens of Syrians from queuing to beg Turkish authorities to allow in their relatives from the other side.

Syrians are feeling an intensifying Russian assault around the city of Aleppo, and aid workers said on Friday it could soon fall under a full government siege.

Turkish Prime Minister Ahmet Davutoglu has said 15,000 people fleeing Aleppo had arrived at Turkey's border, but some media reports put that number significantly higher.

Sitting in his car with his four children right inside the Turkish side of the border, 43-year-old Ahmet Sadul was hoping to get back into Syria so he could find his relatives. A native of Syria's Azaz, he now lives in Kilis.

"Now there are thousands of people from Azaz all waiting on the other side. They escaped from Russians. I want to go and take my relatives. They are bombing Syrians all the time."

"Many people have left Aleppo. But still there are many civilians there. If Russia is successful, we are all dead."

Russia's intervention with air strikes to help longtime ally Syrian President Bashar al-Assad has tipped the war in Damascus' favour, reversing gains rebels made last year.

Turkey, which has taken in more than 2.5 million refugees fleeing the Syrian war, wants Assad out and says only his removal can bring long-term peace.

Ankara has had an open-door policy to refugees since the start of the civil war, although some of the borders do get shut at times for security or other reasons. Oncupinar has been officially shut since March due to security concerns, although authorities occasionally open it to take in refugees.

Abdulkerim Hannura, a 32-year-old customs police officer who works on the Syrian side of the border, said Russian warplanes had been bombing Syrian villages for 15 days.

"People are coming to the border and want to cross into Syria with the hope that they can sneak their relatives back into Turkey," he said.

"We are trying to save our relatives, our families."