Outbreaks of the water-borne disease occur regularly in Congo, mainly due to poor sanitation and a lack of access to clean drinking water.
But this year's epidemic, which has already hit at least 10 urban areas including the capital Kinshasa, is particularly worrying as it comes as about 1.4 million people have been displaced by violence in the central Kasai region.
The WHO said at least 528 people had died and the epidemic had spread to 20 of Congo's 26 provinces.
"The risk of spread remains very high towards the Grand Kasai region, where degraded sanitary and security conditions further increase vulnerability in the face of the epidemic," the WHO said in a statement.
So far, health officials have recorded more than 24,000 suspected cases of the disease across the vast nation this year, averaging more than 1,500 new cases per week since the end of July.
The WHO sent a team of experts including epidemiologists and public health specialists to Congo this month in an effort to contain the disease's spread.