More than one million children, some without parents or close relatives, are among 2.1 million refugees who have crossed mainly into Jordan, Iraq, Lebanon and Turkey since March 2011, the agency says.
Jordan hosts 540,000 Syrian refugees, straining health and education services and already scarce water resources, said Michele Servadei, UNICEF's deputy representative in Jordan.
Syria militants are also alleged to have infiltrated refugee camps in Jordan seeking to recruit young people to fight in their homeland, Servadei said.
Most Syrians live in host communities in the north, while 120,000 are at the teeming Zaatari camp in the Jordanian desert.
"In host communities they are much more exposed to child labor, to early marriage, to exploitation in general," Servadei told a news briefing in Geneva.
Some 200,000 Syrian refugees in Jordan are school-age, but only 80,000 are enrolled in education, often in classrooms with double shifts.
Adolescents aged 14 to 17, many of whom had dropped out of school, were especially at risk, he said.
"The main coping mechanism that these children have in many cases is withdrawal...We noticed that actually many children don't go out of the house," he said.
"But the problem is that the house is not the safest place always. There is a high level of domestic violence among communities, definitely because of the war situation, but also because of the protracted displacement and the sense of frustration that it brings."
Jordan lacks enough shelters for battered women, he added.
UNICEF operates 80 child-friendly spaces in Jordan, offering activities and psycho-social support to young Syrian refugees, some of whom suffer from post-traumatic stress disorder.
An estimated 30,000 Syrian child refugees are working in Jordan, Servadei said. A UNICEF assessment in the Jordan Valley in April identified 3,500 child laborers, mainly seasonal.
"They were working mainly on the farms, in many cases also hard labour, let's say 10 hours a day using pesticides," he said. Other children work in family bakeries or as mechanics.
UNICEF is providing cash assistance - 30 Jordanian dinars or about $45 per month - for families to remove a Syrian child from work and return him to school, according to Servadei.
"We monitor the attendance, when the attendance is no longer there, the cash assistance gets stopped," he said. "But we are checking if that is going to be enough because actually most of these children are earning much more working, unfortunately."
In 2012, 18 percent of the registered marriages of Syrians in Jordan involved under-18-year-olds, up from 12 percent a year before, he said.
Syria crisis started as pro-reform protests but with interventions by the United States, UK and their regional and western allies it soon turned to a massive insurgency which took in numerous terrorist groups from all over Europe and the Middle East to wage one of the bloodiest wars the region has ever experienced.
According to the United Nations, over 100,000 people have been killed and millions displaced in the violence.
SHI/SHI