High Commissioner for Refugees Antonio Guterres spoke as the international community sharply urged Syria to comply with a new Security Council resolution demanding that President Bashar Assad and the opposition provide immediate access for humanitarian aid.
Opposition activists say more than 140,000 people have died in the conflict, which enters its fourth year next month. The UN says 9.3 million Syrians are in need of humanitarian assistance.
The number of Afghan refugees was 2.6 million at the end of 2012, UNHCR says. Syrians, with nearly 2.5 million registered as refugees, should overtake that long before the end of the year. About one-half of the refugees are children.
"It breaks my heart to see this nation that for decades welcomed refugees from other countries ripped apart and forced into exile itself," Guterres told the U.N. General Assembly. Just five years ago, Syria hosted the world's second-largest number of refugees, he said.
Syria's neighbors now plead for assistance as hundreds or thousands of people flee into their countries every day.
The number of Syrian refugees now registered in far smaller Lebanon, for example, is the equivalent of having 71 million of them registered in the United States or almost 15 million in France, Guterres said.
Top UN officials offered a bleak outlook on the overall humanitarian crisis Tuesday as pressure mounted on Syria and the opposition to comply with the new Security Council resolution. The legally binding measure threatens "further steps" if the resolution's demands aren't fulfilled.
The European Union warned that "deliberate denial of humanitarian aid is a war crime." UN High Commissioner for Human Rights Navi Pillay, Britain's UN Ambassador Mark Lyall Grant and others demanded that the Syria situation be referred to the International Criminal Court, which investigates war crimes, crimes against humanity and genocide.
U.S. Ambassador Samantha Power said the number of Syrians needing aid has grown by one-third since the Security Council issued a nonbinding presidential statement in early October on the humanitarian crisis.
"Unfortunately, history teaches us to be skeptical that the terms of this resolution will be observed," she said.
Syria's UN ambassador, Bashar Ja'afari, insisted that his government "has spared no effort" in addressing humanitarian needs and is "perfectly capable of dealing with our own crisis."
UN Secretary-General Ban Ki-moon said the Syrian government should allow more humanitarian workers into the country, which UN officials have called a major issue in reaching an estimated 3.3 million people in isolated areas.
"It is not credible to cite bureaucratic procedures as reasons for delay when it is the government itself that controls those procedures," he said.
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