Oncupinar (Turkey) (AFP) - Turkish humanitarian groups set up camps in Syria and sent in truckloads of aid Monday for tens of thousands of people stranded on the border after fleeing a Russia-backed regime offensive on the northern region of Aleppo.

Turkey, which has long pushed for a safe zone on the border, has vowed to help an estimated 35,000 people amassed on the frontier, many of them women and children.

But so far it has has kept the border closed despite warnings from aid groups of the desperate situation the Syrians were facing.

"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."

Aid agencies have warned of a desperate situation among the crowds queueing in the cold and rain at Bab al-Salama frontier post, which faces Turkey's Oncupinar crossing, and begun setting up camps on the Syrian side.

Many refugees are reportedly sleeping in fields and on roads.

The Turkish Humanitarian Relief Foundation, which is providing food for 20,000 refugees, said it had set up a new camp with a capacity of 10,000, in addition to eight it already operates near Bab al-Salama.

"Our operations are aimed at taking care of people inside Syria," Serkan Nergis, a spokesman for the foundation, told AFP by phone.

"The numbers could soar and we are looking at how we can provide shelter for Syrians in safe areas."

Despite already hosting over 2.5 million refugees from Syria's civil war, Turkey has come under pressure to take in those fleeing the latest flurry of bombardments, while also being squeezed by European leaders to prevent those who cross into Turkey continuing on across the Mediterranean to Europe.

A Turkish official said the border crossing was open only "for emergency situations," adding that several injured people have been taken for treatment to Turkish hospitals.

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- Turkey will 'let brothers in' -

Dozens of aid trucks were seen crossing into Syria on Monday, along with medical teams and ambulances.

Mohammad Rahma, a 15-year-old who was blinded in a Russian air strike a month ago and wore bandages on his eyes, was among those allowed cross into Turkey for treatment, accompanied by his father Ahmad.

"We've been living out in the open because we don't have any place to stay," Ahmad, who came from Azaz, about 30 kilometres from Aleppo city, told AFP.

German Chancellor Angela Merkel, whose country has received over one million migrants in the past year, many of them Syrian refugees, was due in Ankara Monday to press the government to stem the flow of migrants to Europe.

Merkel was to hold talks with President Recep Tayyip Erdogan, who has warned that Turkey is "under threat" from the refugee tide but said that "if necessary, we have to, and will, let our brothers in".

Foreign Minister Mevlut Cavusoglu said Saturday that Turkey had already received 5,000 people fleeing the Aleppo offensive and another 50,000 to 55,000 were on their way.

Medical aid agency Doctors Without Borders (MSF) said three MSF-supported hospitals in the Aleppo region had been bombed.

- 'Situation 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 EU has promised three billion euros ($3.3 billion) of aid in return for Ankara's help in staunching the flow of migrants landing on Greece's shores in packed boats from nearby Turkey.

But residents of areas along Turkey's border with Syria -- where most of the Syrian refugees in Turkey are living -- fear being overwhelmed by a fresh wave of new arrivals.

"Life here would be paralysed in the face of a mass exodus," said Tugba Kaya, from the border town of Kilis.

- 'Survival and dignity' -

Top diplomats from countries trying to resolve Syria's five-year conflict, which has claimed 260,000 lives and displaced half the population, are set to meet pm February 11 after peace talks collapsed last week.

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 Tal Rifaat -- one of the last rebel strongholds in Aleppo province, 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 350,000 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.