Its report "No Place for Children" said more than 8 million children in Syria and neighbouring countries needed humanitarian assistance, with the international response plan for Syria chronically underfunded.
"Twice as many people now live under siege or in hard-to-reach areas compared with 2013. At least 2 million of those cut off from assistance are children, including more than 200,000 in areas under siege," it said.
The UN says more than 450,000 people are under siege. Cases of starvation have been reported this year in areas surrounded by government forces and their allies near Damascus, and by Islamic State in eastern Syria.
Violence continues despite a fragile cessation of hostilities reached last month.
UNICEF said 400 children were killed in 2015. A separate report on Friday by a number of aid groups, including Oxfam, said UN figures showed at least 50,000 people had been killed since April 2014.
"A trend of particular concern is the increase in child recruitment," UNICEF said.
"Children report being actively encouraged to join the war by parties to the conflict offering gifts and 'salaries' of up to $400 a month."
Since 2014, warring sides have recruited younger children, it said, some as young as seven. More than half of children recruited in cases UNICEF verified in 2015 were under 15.
Children have been filmed executing prisoners in grisly propaganda videos by the Islamic State group.
Outside Syria, 306,000 Syrian children have been born as refugees, it said. UN refugee agency UNHCR says nearly 70,000 Syrian refugee children have been born in Lebanon alone.
UNICEF said 3.7 million children had been born since the conflict began, a third of all Syrian children.
Some 2.8 million Syrian children in Syria or neighbouring countries are not attending school. Dozens of schools and hospitals were attacked in 2015, according to aid groups.
"Half of all medical staff have fled Syria and only one third of hospitals are functional. Each doctor used to look after the needs of around 600 people - now it's up to 4000," UNICEF said.
Syria's neighbours host the vast majority of its 4.8 million refugees. Europe hosts an eighth of the number residing in those countries, it said. The separate joint aid agency report, "Fuelling the fire", criticised world powers including Russia, Britain, the United States, France, Turkey, Qatar, Saudi Arabia and Iran, which had "intensified their military engagement in Syria". "To varying degrees, these states - which should play a key role in ending the suffering in Syria - are actively contributing to that very suffering," it said.
UN-brokered peace talks open on Monday in Geneva to seek an end to a conflict that has killed more than 250,000 people.
- SYDNEY MORNING HERALD
