UNITED NATIONS — Syrians could soon overtake Afghans as the world’s biggest refugee population, with their numbers expected to pass 4 million by year’s end, a top U.N. official said Tuesday.

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 U.N. 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 U.N. 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.