Tens of thousands of Syrian refugees came to Syria's border with Turkey on Monday escaping the Syrian regime offensive in Aleppo which is backed by Russian air strikes.
Turkey has vowed to help the crowds of people, including many women and children, providing food and other aid on the Syrian side of the border.
"Turkey has reached the limit of its capacity to absorb the refugees," Deputy Prime Minister Numan Kurtulmus said in a televised interview talking about the new wave of refugees.
The country hosts more than 2.5 million Syrian refugees, the most in the world according to registration records of the United Nations.
"But in the end, these people have nowhere else to go. Either they will die beneath the bombings... or we will open our borders,” Kurtulmus said.
"We are not in a position to tell them not to come. If we do, we would be abandoning them to their deaths," he also said.
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 camps waiting for tents to be distributed by aid agencies.
The governor of Turkey's border province of Kilis, Suleyman Tapsız said, "Our doors are not closed, but at the moment there is no need to host refugees inside our borders."
He added that the refugees had been given food, blankets and tents already on the Syrian side of the border.
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 on Saturday that Turkey had already received 5,000 fleeing the offensive and another 50,000 to 55,000 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.
Situation for refugees highly 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."
Aid trucks and ambulances entered Syria from Turkey on Sunday to deliver food and supplies to tens of thousands of people fleeing an escalating regime assault on Aleppo, as air strikes targeted villages on the road north to the Turkish border.
Turkish humanitarian agencies are also setting up temporary shelters and delivering food to Syrians who have amassed at the border.
German Chancellor Angela Merkel, whose country let in more than a million asylum seekers last year, is due to hold talks with President 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.
Turkey has already spent about $10 billion of its own resources for the Syrian refugees since the war started in 2011.
Inside Turkey, which has the largest population of Syrian refugees with more than 2.5 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 30,000-35,000 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.
Syria talks set to Feb 11
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 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 regime forces closed in on Aleppo city in their most significant advance since Russia intervened in September in support of Bashar al Assad.
Regime troops advanced on Sunday towards the opposition town of Tal Rifaat, around 20 kilometres (12 miles) from the Turkish frontier, a monitoring group said.
It is one of the last opposition 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 opposition groups are now threatened with collapse after the regime severed their main supply line to Aleppo city.
Opposition forces and roughly 350,000 civilians inside opposition-held parts of the city face the risk of a regime siege, a tactic employed to devastating effect against other former opposition bastions.