Cholera outbreak hits war-torn Yemen
"This outbreak adds to the misery of millions of children in Yemen," UNICEF Yemen representative Julien Harneis said in a statement.
"Children are at a particularly high risk if the current cholera outbreak is not urgently contained especially since the health system in Yemen is crumbling as the conflict continues."
The WHO, citing Yemeni health ministry figures, said that eight cholera cases mostly involving children had been recorded in one neighbourhood of the rebel-held capital Sanaa.
They were being treated for extreme dehydration in an isolated section of the capital's al-Sabiine hospital, it said.
The WHO said the scarcity of drinkable water has worsened the hygiene situation in Yemen, fuelling a marked increase in cases of severe diarrhoea, in particular among people displaced from their homes in the centre of the country.
UNICEF said health professionals in Sanaa had reported several cases, as had medics in Taez, Yemen's third largest city.
It said its team was working with doctors in Yemen to establish the cause of the outbreak and called on international donors to provide funding to work to improve the health situation there.
UNICEF said that cholera, a disease that is transmitted through contaminated drinking water and causes acute diarrhoea, could prove fatal in up to 15 percent of untreated cases.
The agency says nearly three million people in Yemen are in need of immediate food supplies, while 1.5 million children suffer malnutrition, including 370,000 enduring very severe malnutrition that weakens their immune system.
The conflict between Yemen's government and the Houthi rebels escalated last year with the intervention of a Saudi-led coalition in support of President Abedrabbo Mansour Hadi.
The UN says more than 6,700 people have been killed and more than three million displaced by fighting in Yemen since March 2015, when the coalition launched its campaign.