Indonesia has seen a major decline in the number of child brides, but a significant 22 percent of Indonesian women aged 20 to 24 were married before the age of 18, according to a landmark report from a United Nations agency.

The report "Marrying Too Young," released on Thursday by the UN Population Fund (UNFPA) to mark the inaugural International Day of the Girl Child, warned that 142 million girls worldwide could be married before 18 within the next decade if global trends continued.

The report noted that during the period from 2000 to 2011, an estimated 34 percent of women aged 20 to 24 years in developing regions were married or in union before their 18th birthday.

"In 2010 this was equivalent to almost 67 million women. About 12 percent were married or in union before age 15," it said.

It also identified Indonesia as one of 48 countries worldwide where the prevalence of child marriages had declined by more than 10 percent in recent years, but noted that the prevalence still remained high, particularly in rural areas.

A large part of the problem in Indonesia was attributed to the fallout of the December 2004 tsunami that killed an estimated 170,000 people in the province of Aceh.

"Girls' vulnerability to child marriage can increase during humanitarian crises when family and social structures are disrupted. In times of conflict and natural disaster, parents may marry off their young daughters as a last resort, either to bring the family some income in time of economic hardship, or to offer the girl some sort of protection," it said.

"Young girls were married to 'tsunami widowers' in Sri Lanka, Indonesia and India as a way to obtain state subsidies for marrying and starting a family."

Data compiled from 2000-2011 showed that 22 percent of women in Indonesia aged 20-24 years were married before 18. The prevalence was higher in rural areas.

Lack of education was also identified as a major factor behind child marriages, with 75.3 percent of girls who got married before 18 having only a primary school education or no education at all. "Girls' education, especially at the secondary level, is strongly associated with later marriage," the UNFPA said.

"Research has suggested that girls with secondary schooling are up to six times less likely to marry while children, compared to girls with little to no education."

Poverty was another factor, with 56.9 percent of Indonesian child brides coming from the two poorest quintiles for household wealth.