Since August 23, at least 6,000 people have been reported with diarrhoeal diseases in Sulaymaniyah province, almost 7,000 in Kirkuk province and at least 3,000 in Erbil province, WHO spokeswoman Fadela Chaib told journalists.
The outbreak has so far remained confined to the three northeastern provinces, which have a population of three million people.
A UN team is working on the ground to help Iraqi authorities fight the epidemic and inspect water sources in the affected areas since the end of August.
The source of the epidemic remains unclear but the WHO remains confident it can be brought under control, Chaib said.
Naeema Al-Gasseer, the WHO's representative in Iraq, warned however that delays in ensuring access to safe water, safe food and enhanced hygiene practices could all lead to the further spread of cholera.
Northern Iraq suffered an earlier cholera outbreak in 1999, while the disease struck the southern province of Basra shortly before the US-lead invasion of 2003.