The report, by the UN Office for the Coordination of Humanitarian Affairs (OCHA), said more than 785,000 children are severely malnourished in Ethiopia, Kenya, Somalia and Uganda due to a dry spell in the region.
"Drought also exacerbates the risk of infectious diseases such as cholera, typhoid fever, diarrhea, acute respiratory infections and measles," OCHA said in the report.
The UN agency said many of the areas impacted by drought across the region have also been affected by internal violence and conflict.
Escalating food insecurity is also likely to further heighten risks for women and girls, who will have to travel longer distances to reach clean water and food.
OCHA warned that the number of food insecure people is expected to rise in the months ahead as a result of consecutive poor seasons that have destroyed livelihoods and eroded the ability of communities to cope with.
According to the UN, rainfall between March and mid-May was less than 50 percent of the annual average across the Horn of Africa.