More than four million Syrians have fled the country since the civil war broke out in 2011. This is the highest number of people to flee a conflict since the Afghan civil war forced 4.6 million out of their country beginning in 1992.

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A Syrian Kurdish refugee woman and her child wait after crossing the Turkish-Syrian border on September 27, 2014 Murad Sezer/Reuters

"This is the biggest refugee population from a single conflict in a generation," Antonio Guterres, UN high commissioner for refugees, said. "It is a population that needs the support of the world but is instead living in dire conditions and sinking deeper into poverty."

The flow of refugees is accelerating – just 10 months ago, the UN agency put the number at three million. The agency said 7.6 million additional people have been displaced from their homes within Syria by the fighting.

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A child who has been living in Jordan for more than two years after fleeing the violence in Idlib, poses in front of his family's residence in Madaba, near Amman, on July 9, 2015 Muhammad Hamed/Reuters
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A Kurdish refugee boy from the Syrian town of Kobani looks out from his tent in a camp in the southeastern Turkish town of Suruc, on October 11, 2014 Umit Bektas/Reuters
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Islamic State fighters laugh as they tell fleeing Syrians to stop trying to cross the Turkish border and go back to the embattled town of Tal Abyad, on June 13, 2015 Bulent Kilic/AFP
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A Syrian refugee fleeing Isis fighters in Tal Abyad holds onto his daughter as he waits to cross into Turkey at Akcakale border gate, on June 15, 2015 Umit Bektas/Reuters

Turkey has borne much of the impact. There are more than 1.8 million Syrians in Turkey, making it the biggest host of refugees in the world, an expensive undertaking that Turkey is bearing mostly on its own. It now fears fighting around the northern Syrian city of Aleppo could push as many as one million more over its borders.

Turkey's EU affairs minister warned it would struggle to cope with a new influx of refugees from Syria's civil war. Volkan Bozkir said many of them would likely end up trying to get into Europe.

He said the amount the country had spent on refugees dwarfed the contribution from the European Union, which Turkey wants to join. "We have spent $6bn so far. The total amount that the EU has provided is €70m and it is still just a promise, it has not yet arrived with us," he said.

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A Syrian Kurdish girl looks at the police as she crosses the border into Turkey on September 21, 2014 Bulent Kilic/AFP
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A Syrian Kurdish man pours water on a child after they crossed the border into Turkey on September 20, 2014 Bulent Kilic/AFP
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A Syrian refugee woman, holding her newborn twins, crosses into Turkey on June 16, 2015 Umit Bektas/Reuters
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Kurdish refugee girls from the Syrian town of Kobani walk at a refugee camp in the Turkish border town of Suruc, on February 2, 2015 Umit Bektas/Reuters
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A Syrian refugee girl sits with her brother at a makeshift settlement in Bar Elias in the Bekaa valley, Lebanon, on January 5, 2015 Mohamed Azakir/Reuters
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A Kurdish refugee woman from the Syrian town of Kobani holds a child in a camp in Suruc, Sanliurfa province, Turkey, October 25, 2014 Kai Pfaffenbach/Reuters
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A Syrian refugee girl poses in a makeshift shelter near a truck close to the Turkish border post of Akcakale, province of Sanliurfa, on June 17, 2015 Gokhan Sahin/Getty Images
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Fatemah Aldan holds her two-year-old brother Saif, who lost both legs during a barrel bomb attack on their home town of Aleppo, as they wait for prosthetic legs to be made at a clinic in Reyhanli, Turkey, on March 24, 2015 Carl Court/Getty Images
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Syrian Kurdish refugees gather around fire in a refugee camp in Suruc, Sanliurfa province, Turkey, on November 11, 2014 Aris Messinis/AFP
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Syrian refugees laugh during a snow storm at an unofficial camp on the road between Riyaq and Baalbek in Lebanon's eastern Bekaa Valley, on January 7, 2015 AFP
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Syrian Kurdish children hold on to a fence in a United Nations Refugee Agency refugee camp at Suruc in Turkey, on February 2, 2015 Bulent Kilic/AFP
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A derelict building housing refugees from Syria is pictured in the Suleymaniye district of Istanbul, on March 19, 2015 Carl Court/Getty Images
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Displaced Syrians try to repair a damaged tent after heavy rains in the Bab Al-Salama camp on the border with Turkey, on December 11, 2014 Baraa al-Halabi/AFP
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An aerial view of the Zaatari refugee camp, near the Jordanian city of Mafraq, on July 18, 2013 Mandel Ngan/AFP

Some 630,000 Syrians are now living in Jordan, while there are more than a million in Lebanon, according to the UNHCR figures.

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