Unicef says the practice inflicts a tremendous physical and emotional toll on young women.

Young brides tend to be pulled out of school, depriving them of their basic right to education, and early marriage almost always means premature pregnancy with added health risks.

Rights violated

"Forcing children, especially girls into early marriages, can be physically and emotionally harmful," Unicef executive director Carol Bellamy said.

"It violates their rights to personal freedom and growth."

Parents may feel that marrying off a daughter at a young age can help them economically, while also keeping the girl "safe" from unwanted sexual advances, the report said.

There is also the false belief in many countries that a man can be cured of Aids by sleeping with a virgin.

In Nepal, 7% of girls are married before they are 10, and 40% by age 15.

In Egypt, studies show that 29% of young brides have been beaten by their husband or other relatives.

Domestic violence often causes girls to run away, and in several countries this can trigger "honour killings" by male relatives eager to wipe out a perceived disgrace to the family.

Unicef says early marriage constitutes a violation of a girl's human rights, primarily because it can deprive her of the right to give full and free consent to marry.

Unicef is calling on governments and local groups to discourage child marriages by educating parents and young people about the implications.

It is also advocating national campaigns to raise the legal age of marriage.