Warning that inaction could lead to a ten-fold rise in the number of people infected, the UN called on donors to urgently fund its emergency appeal to help Haiti deal with a cholera epidemic.
"On the appeal for 164 million dollars, we are today still at 19.4 million dollars," said Elisabeth Byrs, spokeswoman for the UN Office for the Coordination of Humanitarian Affairs.
"It's very slow. We must continue to be vigilant to avoid having to deal with the worst-case scenario as seen by the World Health Organisation of around 400,000 people who could be affected," she added.
Haitian authorities said the cholera epidemic, which began in mid-October, has killed 1,603 people and left around 30,000 hospitalised.
The WHO had previously planned for a worst-case scenario of 200,000 cases, but the UN said that the epidemic has developed more quickly than expected.
The Pan American Health Organization, the regional branch of the WHO, has warned the number of cases could climb to 200,000 in three months, and that within a year, this figure could rise to 400,000.
"We can avoid this worst-case scenario. It is avoidable with a rapid financing of 164 million," said Byrs, stressing the urgency of containing the epidemic.
The UN also said that aid agencies needed new treatment centres urgently. Rehydration centres, trained personnel and relief supplies were also required, it added.