Some 162 million children are left stunted by chronic malnutrition and 99 million children are underweight across the world, it added.
Since the first international conference on nutrition in 1992, "important advances in the fight against hunger and malnutrition have been made, but this progress has been insufficient and uneven," FAO head Jose Graziano da Silva said.
There has only been a 17 percent reduction in undernourishment since the early 1990s, leaving over 840 million people still chronically undernourished.
The FAO and WHO urged governments to "make stronger commitments... to ensure healthier diets for all" at an international conference on nutrition to be held in Rome in November.
They warned that various forms of malnutrition often overlap and can coexist within the same country and even within the same household.
"Around 160 million children under five are stunted or chronically malnourished, while over two billion people suffer one or more micronutrient deficiencies," they said in a statement.
"At the same time, another half billion are obese."
WHO Director-General Margaret Chan said the aim of the upcoming conference was to get governments to ask themselves "why is it that severe undernutrition and obesity can exist side by side in the same country and in the same community?"
She also called for more research into the health and environmental implications of "the rapid rise in the demand for meat and other animal products that coincides with rising income levels."
Efforts to improve food and nutrition security continue to be hampered by low political commitment and weak institutional arrangements, the agencies said.