BEIRUT (AP) — In Syria, last year was the worst yet for the country's rising generation, with at least 652 children killed in 2016, the United Nations' child relief agency said Monday.

There was no letup to attacks on schools, hospitals, playgrounds, parks and homes as the Syrian government, its opponents and the allies of both sides showed callous disregard for the laws of war.

UNICEF said at least 255 children were killed in or near schools last year and 1.7 million youngsters are out of school. One of every three schools in Syria is unusable, some because armed groups occupy them. An additional 2.3 million Syrian children are refugees elsewhere in the Middle East.

The figures came in a UNICEF report released ahead of the sixth anniversary later this week of the 2011 popular uprising against President Bashar Assad's rule. The uprising, which was part of the Arab Spring movements across the Mideast, quickly escalated into full-blown civil war.

Children were among the first victims of the government's brutal crackdown.

On March 15, 2011, a small demonstration broke out in the capital of Damascus and three days later, residents in the southern Syrian city of Daraa marched to demand the release of teenage students arrested for writing anti-government slogans on their school's walls. They were tortured in detention.

The UNICEF report warns that for Syria's young generation, coping mechanisms and medical care are eroding quickly, driving children into child labor, early marriage and combat. Dozens of children are also dying from preventable diseases.

A report released a week ago by the international charity Save the Children said Syrian youngsters are showing signs of "toxic stress" that can lead to lifelong health problems, struggles with addiction and mental disorders lasting into adulthood.

The use of child soldiers is on the rise in Syria, UNICEF also said. At least 851 children were recruited by armed factions last year — more than twice compared to the year before.

Children across the country are at risk of severe injury while playing around land mines and cluster munitions. Demining operations in opposition-held areas have been severely hampered by inaccessibility to outside experts.

Meanwhile, the Norwegian Refugee Council said that as the sixth year of Syria's conflict nears its end, 13.5 million people remain in need of aid in dire and deteriorating conditions. Half as many are displaced in their own country, with almost 5 million refugees in neighboring countries where conditions keep getting increasingly desperate.

"Over the last year in Syria, all parties involved have blocked vital aid supplies and millions have become poorer, hungrier and more isolated from assistance and from the world," said NRC's Mideast director, Carsten Hansen.

"We join the rest of the international humanitarian community on this milestone of shame to voice outrage at the plight of millions of civilians living in a downward spiral of despair," the organization added.

It said parties to the conflict continue using siege and starvation as a weapon of war. Around 5 million people remain trapped in areas of active fighting, including almost one million in besieged areas who have no access to sustained humanitarian assistance.

Associated Press Writer Bassem Mroue in Beirut contributed to this report.