As many as two million children have had their lives torn apart by the bloody conflict in Syria as the fighting enters its third year with no end in sight, Save the Children has warned.
These "forgotten victims" are at constant risk of malnutrition, disease, trauma and, in the case of girls, being married off at a young age in a bid to protect them from burgeoning sexual violence, it said in a new report.
Ahead of talks starting in London on Wednesday between foreign secretary William Hague and his Russian counterpart Sergei Lavrov, which are likely to be dominated by the conflict, the charity urged the international community to push for an end to the violence that has torn Syria apart.
It is planning to hold vigils in 21 countries on Thursday to mark the second anniversary of the start of fighting in the country.
Jasmine Whitbread, Save the Children's international chief executive, said: "For millions of Syrian children, the innocence of childhood has been replaced by the cruel realities of trying to survive this vicious war.
"Many are now living rough, struggling to find enough to eat, without the right medicine if they become sick or injured.
"As society has broken down, in the worst cases, hunger, homelessness and terror have replaced school for some of these young people.
"We cannot allow this to continue unchecked; the lives of too many children are at stake."
Childhood Under Fire, launched to mark two years of fighting in the Middle Eastern country that has claimed 70,000 lives, says many children are struggling to find enough to eat.
Thousands are living in barns, parks and caves and are unable to go to school because teachers have fled and schools have been attacked.
Young boys are also being used by armed groups as porters, runners and human shields, bringing them dangerously close to the frontline, it warns.
The report, which includes research carried out amongst refugee children by Bahcesehir University in Turkey, says one in three children reported having been hit, kicked or shot at during the conflict.
Three in every four Syrian children interviewed had lost a loved one because of the fighting. Children are being killed and maimed too, it warned, including by the indiscriminate use of shells, mortars and rockets.
Thousands face malnutrition as food production is crippled and damage to sanitation systems has raised the risk of disease, it says.
Girls are being married off early to ensure that they have someone who can protect them from sex attacks.
And fuel prices have risen by up to 500%, leaving families struggling during the winter.
The prospects for peace currently look dim. Last week Mr Lavrov told the BBC there was "absolutely" no prospect of Moscow urging Syrian president Bashar Assad to stand down.
He said the international community should drop any "preconditions" for negotiations between the government in Damascus and the rebels behind a two-year uprising against Assad's rule.
Mr Hague also announced that Britain would send armoured vehicles and body armour to Syrian opposition forces as it steps up efforts to end a humanitarian crisis of catastrophic proportions.
Save the Children said that $1.5 billion pledged in aid needs to be delivered to those suffering in the country and in refugee camps in neighbouring countries, with some areas still not having received any foreign aid.