The UN's health agency said the epidemic had left more than 25,000 people sick, warning that number was likely to double by the end of June.
WHO put the number of deaths since the beginning of the year from the epidemic at 524, while the UN humanitarian agency OCHA said 533 people had died.
The case fatality rate, which measures the severity of an epidemic by defining the proportion of fatal cases within a specific timeframe, is currently 2.1 percent -- more than double the emergency threshold of one percent.
OCHA spokesman Jens Laerke said the situation was particularly alarming in the Middle Juba and Bakool regions, where the case fatality rates have surged to 14.1 percent and 5.1 percent respectively.
The epidemic comes as the Horn of Africa country of 12 million people faces the threat of its third famine in 25 years of civil war and anarchy.
At least 260,000 people died in the 2011 famine in Somalia -- half of them children under the age of five, according to the UN World Food Program.
Currently, 6.2 million people are in urgent need of humanitarian aid in Somalia, including 2.9 million who are facing "crisis or emergency levels of food insecurity", Laerke told reporters.
That means they are at levels three or four on a five-level scale, where level five is famine.
The dire drought and food situation has forced more than 500,000 people to flee their homes since last November, in a country where 1.1 million people are already internally displaced.
And the drought is not expected to end any time soon, Laerke said, pointing out that the UN expects no improvements over the next six months at least.
Even as conditions deteriorate on the ground, humanitarian actors are struggling to raise enough funds to provide aid, and to gain access to all of those in need, he said.
"We are in a race and I don't know who is going to win," Laerke said.