Turkey has vowed to help the crowds of people including many women and children but has not opened its borders and aid agencies have warned they are facing a "desperate" situation as they wait for help.

"Turkey has reached the limit of its capacity to absorb the refugees" Deputy Prime Minister Numan Kurtulmus told CNN Turk television.

"But in the end these people have nowhere else to go. Either they will die beneath the bombings... or we will open our borders.

"We are not in a position to tell them not to come. If we do we would be abandoning them to their deaths."

Turkey's Oncupinar border crossing which faces the Bab al-Salama frontier post inside Syria remained closed on Sunday as thousands of refugees gathered there for a third day waiting for the gate to open.

Carrying their few belongings Syrians queued in the cold and rain in squalid camps waiting for tents being distributed by aid agencies. Others are reportedly sleeping in fields and on roads.

A Turkish official said the crossing was "open for emergency situations" adding that seven injured people have been taken for treatment in Turkish hospitals.

President Recep Tayyip Erdogan has said Turkey is "under threat" and pledged that "if necessary we have to and will let our brothers in".

Foreign Minister Mevlut Cavusoglu said Saturday Turkey had already received 5000 fleeing the offensive and another 50000 to 55000 were on their way.

The medical aid agency Doctors Without Borders (MSF) said camps for displaced people in northern Aleppo province were overwhelmed and three MSF-supported hospitals had been bombed.

- 'Desperate' -

"From what MSF can see the situation in Azaz district is desperate with ongoing fighting and tens of thousands of people displaced" said Muskilda Zancada head of the group's Syria mission.

"We... have seen problems with lack of space to accommodate people and insufficient water and sanitation in many areas."

The European Union has said Ankara is obliged to keep its frontiers open to refugees while also pressing to help stem the largest influx of migrants into the bloc since World War II.

German Chancellor Angela Merkel whose country let in more than a million asylum seekers last year was due to hold talks with Erdogan and his Prime Minister Ahmet Davutoglu in the Turkish capital on Monday.

The EU has promised three billion euros ($3.3 billion) of aid in return for Ankara's help in stopping the flow of new arrivals on its shores most of whom make their way through Turkey.

Inside Turkey which has the largest population of Syrian refugees with some 2.7 million people on its soil residents of border areas fear being overwhelmed by a fresh wave of new arrivals.

The governor of Kilis province Suleyman Tapsiz said Turkey was taking care of 30-35000 refugees who had gathered around the Syrian city of Azaz in the space of 48 hours.

"Life here would be paralysed in the face of a mass exodus" said Tugba Kaya from the border town of Kilis Turkey's only major urban centre with a majority of Syrians.

- 'Survival and dignity' -

Top diplomats from countries trying to resolve Syria's five-year conflict which has claimed 260000 lives and displaced half the population are set to meet February 11 after peace talks collapsed last week.

Pope Francis urged the international community "to spare no effort to urgently bring parties back to the negotiating table" and appealed for generosity to ensure the "survival and dignity" of displaced Syrians.

The latest crisis began as Syrian government forces closed in on Aleppo city in their most significant advance since Russia intervened in September in support of President Bashar al-Assad.

Regime troops advanced Sunday towards the rebel town of Tal Rifaat around 20 kilometres (12 miles) from the Turkish frontier a monitoring group said.

It is one of the last rebel strongholds in northern Aleppo province and government troops are just seven kilometres away the Britain-based Syrian Observatory for Human Rights said.

Syria's mainstream rebels are now threatened with collapse after the regime severed their main supply line to Aleppo city.

Opposition forces and roughly 350000 civilians inside rebel-held parts of the city face the risk of a government siege a tactic employed to devastating effect against other former rebel bastions.

On Sunday an aid convoy entered the regime-besieged town of Moadamiyat al-Sham near Damascus in a new joint operation organised by the Red Cross and Red Crescent.