The number of girls who marry before their 18th birthday could increase dramatically over the next two decades, a new UN report warned Thursday.

If current trends continue, the tally of such unions will rise to 14.2 million a year in 2020, and 15.1 million each year in 2030, according to the report by the United Nations Population Fund (UNFPA).

Released on the first International Day of the Girl Child, the survey shows that despite efforts to rein in the practice, the frequency of such weddings has remained fairly constant in the developing world over the past decade.

"Child marriage is an appalling violation of human rights and robs girls of their education, health and long-term prospects," said UNFPA executive director Babatunde Osotimehin.

"Marriage for girls can lead to complications of pregnancy and childbirth, the main causes of death among 15-19-year-old girls in developing countries."

In 2010, one in three women -- or 67 million -- aged 20-24 were wed before their 18th birthday in developing countries excluding China.

Roughly half of these unions took place in Asia and another 20 percent in sub-Saharan Africa. The practice also exists in Latin America and the Caribbean, and in Eastern Europe.

In South Asia, Bangladesh has the highest prevalence of child marriage, with 66 percent. In the west African country of Niger, 75 percent of young women aged 20 to 24 were married before turning 18, according to 2010 figures.

If nothing is done to stop the custom, UNFPA estimates that, from now until 2030, 130 million girls in South Asia, 70 million girls in sub-Saharan Africa and 45.5 million girls in Latin America and the Caribbean risk facing a similar fate.

But there is some good news. Certain developing countries -- including Armenia, Bolivia, Ethiopia and Nepal -- have seen a decrease in child marriages. But the practice persists.

"I feel about this as I feel about apartheid," South African Archbishop Desmond Tutu, a Nobel Peace Prize winner, told reporters.

Salamatou Aghali Issoufa, a young Nigerian activist who succeeded in convincing her parents not to marry her off to a 50-year-old man at the age of 14, appealed for girls to be given a chance and granted access to education.

After completing her studies, she now works as a midwife in her village.

"I had a lot of luck, in contrast to my cousin who was married when she was 16 and who already has three children," she said.

The report calls on governments to promote and enforce laws setting the legal age for marriage at 18 years of age for both boys and girls.

In 2010, 158 countries reported that 18 was the minimum legal age for women to get married without parental consent. But in 146 countries, such a union is accepted before this age -- under 15 in 52 countries -- if parents agree to it.

Girls living in rural regions prematurely marry twice as often as their counterparts in urban areas. Those without schooling are three times more likely to do so early on than those who received at least a secondary education.

Humanitarian crises often make young girls more vulnerable because their families may marry them off for a dowry or benefits -- or in the hopes of providing them with shelter.

The report cites "famine brides" who are married off by parents in food-insecure Kenya as a last resort. It also references young girls who are wed to "tsunami widowers" in Sri Lanka, Indonesia and India as a way of obtaining state subsidies for marrying and starting a family.