The civil war in Syria entered its fourth year in March. UNICEF said in a report on the conflict the number of children affected by ongoing violence has doubled since March 2013.
"For Syria's children, the past three years have been the longest of their lives," Lake said in a statement Monday. "Must they endure another year of suffering?"
UNICEF said there may be as many as 1 million children trapped by the fighting inside Syria. Of the 1.2 million children living as refugees outside the country, Lake said about 10 percent of them have entered the labor pool and 20 percent of Syrian girls are forced into early marriage.
"This war has to end so that children can return to their homes to rebuild their lives in safety with their family and friends," Lake said. "This third devastating year for Syrian children must be the last."
The U.N. World Food Program, meanwhile, said it was having a hard time gaining access to civilians in need of assistance as the conflict drags on.
"One-off convoys into besieged areas can provide temporary relief, but WFP still needs proper and sustained access to people to provide life-saving assistance and also to assess the scale of the needs," WFP Deputy Executive Director Amir Abdulla said in a statement.