Russian and Syrian government forces gained ground north of Aleppo on Saturday, intensifying an assault on rebel-held areas that has prompted tens of thousands more Syrians to seek refuge across the border in Turkey.

The assault around Aleppo, which aid workers have said could soon fall to government forces, helped to torpedo Syrian peace talks in Geneva this week. Russia's intervention has tipped the war President Bashar Al Assad's way, reversing gains the rebels made last year.

Aleppo, Syria's largest city before the civil war erupted five years ago, would be a huge strategic prize for Assad's government in a conflict that has killed at least 250,000 people and driven 11 million from their homes.

Advances by the Syrian army and allied militias, including Iranian fighters, threaten to besiege opposition-held areas of the divided city. Government-controlled parts of Aleppo are home to more than a million people, while around 350,000 live in opposition-held areas.

On Saturday about 15,000 Syrians were waiting on the Syrian side of Turkey's Oncupinar border crossing and up to 50,000 more were on their way, an official with the Turkish disaster agency AFAD told Reuters.

"At the moment there isn't an emergency situation in terms of security where the crowd is waiting. The first priority for them was to be in a secure place and the other (Syrian) side of the border is secure," the AFAD official said.

"Food and aid is being distributed. For now there isn't an imminent risk to their lives."

Oncupinar has been officially shut for nearly a year due to security concerns and remained closed on Saturday, but it is opened from time to time to allow refugees into Turkey, which has already taken in some 2.5 million Syrians.

A Reuters reporter at Oncupinar could hear infrequent shelling and witnessed a few Turkish ambulances cross the border at one point.

Dozens of Syrian refugees already in Turkey queued up on the Turkish side at Oncupinar to beg the authorities to allow in their relatives fleeing the latest bombardments in Syria.

Sitting in his car with his four children just inside Turkey, Ahmet Sadul, 43, was hoping to get back into Syria to look for his relatives. A native of Azaz, over the border in Syria, he now lives in the nearby Turkish town of Kilis.

"Now there are thousands of people from Azaz all waiting on the other side. They escaped from the Russians. I want to go and get 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 denies targeting civilians and says its actions are aimed at shoring up Syria's legitimate government and combating terrorism. The West and Turkey, which want Assad to step down, accuse Moscow of using indiscriminate force in the conflict.

The Syrian Observatory for Human Rights monitoring group said Kurdish and Syrian rebel factions in the countryside north of Aleppo had agreed to open a humanitarian corridor from Azaz into Kurdish-controlled Afrin for those fleeing the bombing but unable to cross into Turkey.

The United Nations said up to 10,000 people had been displaced to Azaz from areas under attack north of Aleppo and that 10,000 had been displaced to Afrin, where there are plans to expand an existing camp for internally displaced persons.

But the fighting is making access to populations in need "increasingly difficult", a UN official said.

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.-Reuters