The WHO said in a statement it was not recommending any travel or trade restrictions to and from affected areas in Iraq, where 3,389 cases of cholera have now been confirmed and 14 people have died from the disease.
"However, neighbouring countries are encouraged to reinforce their active surveillance and preparedness systems," it added.
More than 3,100 of the cases are in the northeastern provinces of Kirkuk and Sulaymaniah where the outbreak first emerged in August, but infections were now reported in nine out of 18 provinces, the WHO said.
"The disease is continuing to spread across Iraq and dissemination to as yet unaffected areas remains highly possible," it added.
The disease reached Baghdad two weeks ago, and a growing number of cases of acute watery diarrhoea -- which are suspected to be cholera -- were now being reported in Diyala, a province neighbouring the capital, according to the WHO.
It "strongly discouraged" preventive medication, saying the step had no effect and could increase resistance of the bacteria, while vaccination was not recommended once an outbreak was underway.
The outbreak and its spread was blamed on poor quality water supplies and sanitation.
The UN health agency said it was buying five million water treatment tablets for Iraq, and sending and two epidemiologists to help the Iraqi health ministry fight the cholera outbreak.