"The situation for countless communities across the region... is still desperate," the OCHA factsheet says. "People are highly vulnerable to disease and epidemics, and because health infrastructure is so limited, many people die of illnesses that they should survive."
UN Regional Humanitarian coordinator for the Sahel Robert Piper told the Reuters news agency that the region now faces "make or break time".
In south-east Niger, for example, a convergence of floods, droughts and conflict in neighbouring Nigeria means that people near the town of Diffa cannot grow enough food to get by.
While the Sahel belt often sees floods, droughts, locust infestations and epidemics, it has also been hit more recently by conflicts in several countries including Nigeria, South Sudan and Central African Republic. Across the Sahel, more than 1.2 million people have had to flee violence and insecurity, with most of them crossing into neighbouring countries as refugees where they place further strain on already limited resources.