GENEVA — The United Nations is trying to raise close to a billion dollars to try to pick up the pieces of Syria’s shattered education system and offset the “staggering decline” in what was once one of the region’s leading school systems after nearly three years of conflict.

Among the 4.8 million Syrian children of school age, some 2.2 million who are still in the country are no longer in school, joining at least half a million — and probably many more — child refugees in neighboring countries, the United Nations Children’s Fund said in a report released on Friday.

“We’ve so far focused on lifesaving initiatives, but we also need to look longer term now because the future of Syria will rest on the shoulders of these children,” said Marixie Mercado, a spokeswoman for Unicef in Geneva. Failure to seize this opportunity “will perpetuate the costs of the conflict for decades to come,” she said.

Unicef is seeking a total of $990 million from donors to support the initiative, including some $600 million in an appeal for emergency funding in 2014 and another $390 million it hopes to raise through established development channels.