The United Nations said Tuesday the worsening conflict in Syria has left 13.5 million people in need of aid and some form of protection, including more than six million children.

U.N. humanitarian chief Stephen O'Brien told the Security Council that the figure represents an increase of some 1.2 million people in just 10 months, and with winter fast approaching the situation for many families is likely to deteriorate.

"This is one of the largest displacement crises of modern times," he said. "Fighting and violence has forced over half of the people in Syria from their homes in a period of just over four years, many of them multiple times."

O'Brien said there are now some 6.5 million people displaced inside Syria, including well over 1.2 million so far this year and over 120,000 who have fled their homes in the country's north since early October as a result of aerial bombardment and ground offensives. Russia's aerial campaign in support of Syrian President Bashar Assad's government has enabled government troops to launch a new campaign in the north.

O'Brien said 4.2 million Syrians have fled the country, including an increasing number risking dangerous boat trips to reach Europe. He said over 50 percent of the 680,000 people arriving in Europe via the Mediterranean in 2015 are Syrian.

"They have a right to seek asylum without any form of discrimination," O'Brien added.

He criticized the "contempt" of all combatants for a 2014 Security Council resolution urging fighters to protect civilians and allow unimpeded access for humanitarian workers.

In Aleppo, Syria's largest city, O'Brien said pro-government and opposition forces continue to shell civilian areas, with hundreds killed in September. On Oct. 23, the main access route from Hama to western Aleppo was cut due to an offensive by the Islamic State extremist group, he said.

O'Brien said attacks on health facilities and health workers "remain relentless." Five hospitals in Hama, Idlib and Aleppo have been attacked since the launch of recent offensives, leading to casualties and their closure.

O'Brien said despite its best efforts, the U.N. has only been able to reach a small fraction of the 4.5 million people living in hard-to-reach areas.