The United Nations Children's Fund (UNICEF) has voiced warnings over a global slowdown in momentum to end child marriages.

In an analysis released Wednesday, the U.N. body said that though the number of child marriages globally was slowly declining, conflict and economic hardship could undo the hard-won progress.

UNICEF estimates that 12 million girls enter into child marriages every year. There are currently 640 million girls and women living in the world who were married before their 18th birthday.

The proportion of young women in child marriages has fallen from 21% to 19% since the last estimates five years ago.

However, UNICEF Executive Director Catherine Russell warned: "The world is engulfed by crises on top of crises that are crushing the hopes and dreams of vulnerable children, especially girls who should be students, not brides."

Health and economic crises, escalating armed conflicts and the devastating effects of climate change would force families to seek perceived safety in child marriages, she said.

Sub-Saharan Africa, where the population is booming, is even expected to see an increase in child marriages.

Latin America and the Caribbean, in the meanwhile, is on course to have the second-highest level of child marriage by 2030.

Other regions, like the Middle East and North Africa, as well as Eastern Europe and Central Asia, are seeing their numbers stagnate after years of progress.

Progress in South Asia is largely responsible for the overall positive global trend. However, almost half of all child brides – 45% – still live in the region.

Although India has made considerable progress in recent decades, one-third of the world's child marriages still take place there.

Girls forced into child marriages are less likely to stay in school and are at increased risk of early pregnancy. Early marriage can also isolate girls from family and friends, UNICEF said.