"The sheer number of girls affected - and what this means in terms of lost childhoods and shattered futures - underline the urgency of banning the practice of child marriage once and for all," UNICEF executive director Anthony Lake said in a statement at the start of a two-day African Union summit on ending child marriage.
"Each child bride is an individual tragedy. An increase in their number is intolerable."
The AU launched a campaign earlier this year to end child marriage. The minimum legal marriage age is 15 in about a dozen African countries, and change is gradual.
Just over one in three African girls marry before the age of 18, most commonly in poor, rural families which often receive a bride price or dowry in exchange for their daughter.
The proportion of young women in Africa who married before the age of 18 has dropped to 34 percent now from 44 percent in 1990, but other continents` populations are growing more slowly, and their rates of child marriage are falling faster.
Africa`s population of girls under 18 is predicted to rise from 275 million today - 25 percent of the global total - to 465 million by 2050, 38 percent of the total.
NO CHANGE AMONG POOREST FAMILIES
Virtually no progress has been made among the poorest African families, where the likelihood that a girl will marry as a child is as high today as it was 25 years ago.
In families that struggle to feed, clothe and educate their children, marriage is often seen as the best chance to secure a girl`s future and safeguard her chastity.
"They see child marriage as the best chance to protect their daughters," said UNICEF`s Associate Director for Child Protection, Cornelius Williams.
"If they had access to school, they would have a different perception of their girls -- as income earners, bosses, teachers, medical doctors, lawyers and policewomen. The practice would die naturally."
It is also important to increase girls` access to reproductive health services so that they have fewer, safer pregnancies and can break the cycle of poverty, UNICEF said.
Child brides are more likely to die as a result of pregnancy and childbirth and to be beaten, raped or infected with HIV by their husbands than women who marry later.
Children born to teenage mothers have a higher risk of being stillborn, dying soon after birth and having low birthweight.
African governments also need to make sure that more girls` births are registered so that their age is known, and to enforce laws prohibiting child marriage, UNICEF said.
"We are not seeing the change that is required," Williams said. "We need to accelerate it."
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