An additional 10 million girls may be forced into child marriages before the end of the decade because of the COVID-19 pandemic, UNICEF said on Monday.
The UN's agency for children welfare and rights warned that the pandemic is threatening to undo years of progress in reducing the practice.
"COVID-19 has made an already difficult situation for millions of girls even worse," UNICEF Executive Director Henrietta Fore, said in a statement issued to mark International Women's Day.
"Shuttered schools, isolation from friends and support networks, and rising poverty have added fuel to a fire the world was already struggling to put out," she added.
Before the global health crisis, UNICEF estimated that 100 million girls were at risk of child marriage in the current decade despite "significant" reductions in several countries. Over the previous 10 years, the proportion of young women married as children had decreased by 15 per cent — the equivalent of 25 million marriages averted.
But the projected 10 per cent increase spurred by the pandemic means the "gain is now under threat".
An estimated 650 million girls and women alive today were married in childhood with about half of those marriages taking place in Bangladesh, Brazil, Ethiopia, India and Nigeria.
Girls who marry in childhood are less likely to remain in school and more likely to experience domestic violence, UNICEF said. Child marriages also increase the risk of early and unplanned pregnancy and in turn, the risk of maternal complications and mortality. Isolation from friends and family can also take a heavy toll on the girls' mental health and well-being.
UNICEF stressed that as "marriage fundamentally alters the course of a girls life, the full effect of the pandemic on human development will play out over a generation."
"One year into the pandemic, immediate action is needed to mitigate the toll on girls and their families," Fore said.
"By reopening schools, implementing effective laws and policies, ensuring access to health and social services — including sexual and reproductive health services — and providing comprehensive social protection measures for families, we can significantly reduce a girl's risk of having her childhood stolen through child marriage," she went on.
Experts told Euronews last month that the pandemic had also led to a rise in the number of girls subjected to female genital mutilation.