Unicef says the number of Syrian children in need has more than doubled in the past year to 5.5 million.
It says more than 10,000 are known to have been killed in three years of conflict, although the real number is probably higher. Children in Syria, it says, have lost "lives and limbs, along with virtually every aspect of their childhood".
The statistics presented in Unicef's report, Under Siege, are daunting.
Up to a million children live in areas that are either under siege or very hard for relief agencies to reach, while about three million have had their education completely disrupted.
More than three million been displaced inside Syria - a threefold increase in the space of a year - and 1.2 million have become refugees abroad, up from 260,000. About 425,000 refugees are under five.
In addition, many children have had to start working early and very young girls have been forced to marry for financial reasons. Boys as young as 12 have been recruited to support the fighting.
About two million children are said to need counselling for trauma.
The Unicef report warns the situation is likely to get worse.