The outbreak has spread fast in 20 out of 22 Yemeni governorates in just seven weeks, since April 27, Xinhua news agency reported on Sunday.
Last month, the WHO said the number of suspected cases could reach 300,000 in the next six months.
The organisation warned that the healthcare system in Yemen is on the verge of collapse as many hospitals have shut down because of the ongoing conflict, saying only 45 per cent of them are still operational and they are facing a shortage of supplies and staff.
Two-thirds of the total population, around 19 million, need humanitarian and protection aid.
About 10.3 million people are close to famine and 14.5 million lack access to safe drinking water.
The war has so far killed more than 10,000 people, half of them civilians, and displaced over 2 million others, according to humanitarian agencies.