The U.N. agency said a breastfed child in a developing nation is nearly three times more likely to survive infancy than a child who is not breastfed.
The report was released Tuesday, at the start of a weeklong commemoration of the most natural way to nourish babies.
"World Breastfeeding Week gives us an opportunity to advocate for a very simple way to save children's lives," said UNICEF Executive Director Ann Veneman. "Though breastfeeding rates are increasing in the developing world, an estimated 63 percent of children under 6 months of age are still not adequately breastfed. As a result, millions of children start their lives at a disadvantage."
World Breastfeeding Week -- observed in more than 120 countries -- promotes exclusive breastfeeding for the first six months of life.
Continued breastfeeding after six months, for up to 2 years of age or beyond, combined with safe and appropriate complementary feeding, is the optimal approach to child feeding, UNICEF officials say.