An increasing number of Syrian refugees in Jordan are marrying off their underage children despite aid workers' efforts to curb the practice, Unicef officials have said.

"A preliminary analysis of 2013 data suggests that the number of early marriages has gone up," Michele Servadei, Unicef deputy representative in Jordan, told IBTimes UK. He was unable to provide an exact figure.

Servadei said that continued displacement of Syrian families and economic hardship caused by the civil war meant that some parents saw dowry money as a possible source of livelihood.

"[Hardship] may lead families to marry off their girls to lessen the economic stress on the family," added Maaike Van Adrichem, Unicef child protection specialist in Jordan.

According to Unicef, in 2012 18% of all marriages between Syrian refugees registered in sharia courts in Jordan involved an underage girl or boy. The number went up in 2013, partly because of a rise in refugee numbers.

Sham marriages, where girls are sold off to elderly foreigners who often abandon them a few weeks later, have also been reported.

Many unions go unregistered due to the differences between the Jordanian and Syrian legal and social framework, and aid workers are struggling to monitor the more than 400,000 refugees living in Jordan's urban areas outside the camps.

Jordanian law allows girls and boys aged between 15 and 18 to marry, but only under exceptional circumstances. In Syria girls are allowed to marry as early as 13.

Early marriage was a common practice before the war, especially in conservative, rural areas. A 2011 Unicef report found that 13% of girls in Syria were married before turning 18.

Conflict, poor safety and precarious living conditions experienced by refugees have resulted in an increase in parents' desire to marry off their children, especially daughters, at a young age as the marital status is perceived as a form of protection.

"A lot of people consider that in a situation of insecurity marriage will provide some kind of safety for her," said Lakshmi Sundaram, global coordinator for NGO Girls Not Brides.

"What they don't necessarily realise is that a marriage can be an incredibly unsafe environment for a girl as she is at much greater risk of domestic violence and sexual violence.

"Data shows that girls who marry young are much more likely to experience violence within the marriage and their first sexual experiences are often forced.

"They can also face difficult health consequences as they often have children long before their bodies are ready," she explained.

Education is one weapon in the fight to reduced underage marriages, Sundaram said. Children who have been educated are six times less likely to marry at a young age. One additional year of secondary schooling increases a girl's earning potential by15-25%, she added.

Unicef said it was working closely with the Jordanian government and civil society partners to develop strategies to reduce the number of early marriages.