It, however, raises particular concern over the situation in sub-Saharan Africa, which it says appears to be bucking the overall trend for declining numbers.

In a new report, Unicef says the number of girls married off before reaching the age of 18 has been declining over the past decade, but that health and economic crises, armed conflict, and the effects of climate change are forcing some parents into marrying off their daughters.

It estimates that around 12 million girls globally become child brides each year, describing the practice as a clear violation of children's rights.