A number of aid agencies said Tuesday 1.4 billion dollars is needed to educate around 4 million Syrian children and young people, a day ahead of a conference due to be held in London.
"Nearly five years into the crisis, around 4 million Syrian and host community children and youth aged 5 - 17 years are in need of education assistance," said a statement from aid groups and UN agencies making up the No Lost Generation initiative.
"These include 2.1 million out-of-school Syrian children inside Syria and 0.7 million ... in Turkey, Lebanon, Jordan, Iraq and Egypt," the statement added.
The No Lost Generation initiative includes the Norwegian Refugee Council, Save the Children, UNICEF, UNRWA and UNHCR.
A conference, due to be held in London Thursday, and attended by heads of state and ministers from countries around the world, aims to raise new funding to meet the immediate and longer-term needs of those affected by the Syrian crisis.
The conference will be co-hosted by Britain, Germany, Kuwait, Norway and the UN.
"The scale of the crisis for children is growing all the time, which is why there are now such fears that Syria is losing a whole generation of its youth," said Dr Peter Salama, regional director for the Middle East and North Africa for UNICEF, the agency coordinating the initiative.
"What we must see in London is the step-change necessary to bring all children back to learning; to protect those who are at risk of dropping out and expand safe and inclusive learning environments," he added.
The combined efforts last year of governments and international partners helped more than 1 million children and young people inside Syria benefit from formal or non-formal learning opportunities, according to the aid agencies.
The agencies urged those attending the London conference to help end attacks on schools and other places of learning, in accordance with International Humanitarian Law.
"In Syria, the killing, abduction and arrest of students and teachers [and] arbitrary attacks on schools have become commonplace," the agencies said.
"About one in four schools cannot be used because they have been damaged, destroyed or are being used as shelters for the internally displaced or for military purposes," the statement added.