Unicef warns that lack of basic services combined with mental distress and poverty could lead to irrevocable loss of potential

The future of 5.5 million children living in Syria and neighbouring countries hangs in the balance as violence, collapsing health and education services, severe psychological distress and impoverishment combine to scar a generation, a UN report has warned.

As the Syrian conflict enters its fourth year, Unicef, the UN agency for children, says the number of children affected has more than doubled, from 2.3 million a year ago. The number of children displaced inside Syria has increased from 920,000 to almost 3 million, while the number of child refugees has risen from 260,000 to more than 1.2 million amid the world's largest humanitarian disaster.

The Unicef report, Under Siege, estimates that 10,000 children have been killed, with up to another million trapped in areas of Syria that are hard to reach with humanitarian assistance due to continued violence.

"For Syria's children, the past three years have been the longest of their lives. Must they endure another year of suffering?" asked Anthony Lake, Unicef executive director.

Another year of conflict could cost far more than lives, warns the report.

"It will mean the irrevocable loss of the skills and understanding they will needs as adults, to play their part in the reshaping of their nation and the restoration of stability to the region. Millions of young people risk becoming, in effect, a lost generation," says Unicef.

Children have increasingly been caught up in the war. Boys as young as 12 have been recruited to support the fighting, some in actual combat while others work as informers, guards or arms smugglers. Families have also described how children are seized by forces from homes, schools, hospitals and checkpoints.

Some children have been subjected to torture and sexual abuse to humiliate them, force confessions, or pressure relatives to surrender. Unicef says there have also been reports of child rapes, including gang rape, and of children used as human shields – forced to the frontlines to stand between tanks and fighters to dissuade enemies from attacking.

Those families that have managed to reach Jordan and Lebanon are under increasing strain, with children having to work to make ends meet. Unicef estimates that one in 10 refugee children is now working and one in every five Syrian girls in Jordan is forced into early marriage.

The report cites the case of Manal, 16, who was told by her father to leave school and marry an older man.

"I felt [my father] was no longer supporting me. I told him I must continue learning," she is quoted as saying. But her father believed a husband would keep Manal safe should anything happen to him.

As refugees flee to Lebanon, Jordan, Turkey and Iraq, those countries are struggling to cope. Their health and education systems are buckling under the strain, as are other services, and tension between Syrians and local communities is increasing.

A World Vision report (pdf) that draws on input from 140 Syrian children in Lebanon and Jordan highlighted problems of bullying and discrimination.

"One of my friends received an intense beating from the principal when he accused my friend of something he had not done," said Mohamed, 14 who attends a school in Lebanon. "When my friend tried to defend himself, the principal did not believe him and hit him on his head until he bled."

Children tell of overworked parents and the effect on family life.

"My friend's father comes home exhausted," said one interviewee. "If my friend tried to talk to him, his father would hit and abuse him."

The children say they cannot talk to their parents because they "are suffering like us and more. At the end, we find ourselves listening to them, instead of them listening to us."