The health ministry in Port-au-Prince has confirmed that the country's cholera outbreak has reached the Haitian capital.
Doctors are treating 73 people for the disease, amid fears that it could spread across the quake-hit city.
Meanwhile, the Pan American Health Organisation (Paho) says it expects tens of thousands more Haitians to catch cholera in the next few years.
The health ministry says so far 583 people have died in the epidemic.
Dozens of suspected cases are also being investigated in Port-au-Prince, which has feared an outbreak since October.
Across the country, more than 9,000 people are being treated for symptoms of the disease, the health ministry said on Tuesday.
"The epidemic of cholera, a highly contagious disease, is no longer a simple emergency, it's now a matter of national security," the director of Haiti's health ministry, Gabriel Thimote, told a news conference.
Officials are warning that a sizeable outbreak in Port-au-Prince is now likely.
The deputy director of the Pan American Health Organisation (Paho), Jon Kim Andrus, said his organisation expected cholera transmission in Port-au-Prince "to be extensive".
Mr Andrus said Paho and other organisations were preparing for a long-running epidemic likely to infect tens of thousands of people across Haiti over the next few years.
"The disease now has a foothold in Haiti's river system," he added.
Mr Andrus said that while it is impossible to predict the development of the epidemic in Haiti, for planning purposes Paho has extrapolated from the 1991 cholera outbreak in Peru, which spread to 16 Latin American countries over six years.
Comparing the population figures of Peru and Haiti and factoring in the number of cholera cases in Peru during the outbreak in the 1990s, the organisation thinks that 270,000 people in Haiti could fall ill.
"But the number in Haiti could be even higher, because the conditions there are worse than in Peru," Mr Andrus told the BBC.
So far the Haitian death rate has also been higher than that registered during the Latin American epidemic, he added.
While trying to educate the population about prevention and treatment of the disease, reducing the fatality rate of the disease in the Caribbean country remained a top priority, Mr Andrus said.
Paho said it was also on "high alert" over the risk of cholera spreading to the neighbouring Dominican Republic and has sent experts to prepare for a possible outbreak in that country.
Mr Andrus warned there could be an "upsurge" in cholera cases in Haiti in the coming days as a result of water and sanitation problems caused by Hurricane Tomas at the end of last week.
"We have every reason to expect that the widespread flooding has increased the risk of cholera spreading."
The water-borne disease has already spread to half of Haiti's 10 regions.
The storm at the end of last week left 20 people dead, with 36 injured and 11 missing, officials said.
Aid agencies say the main concern is that the flooding could result in people lacking access to basic sanitation and being forced to drink contaminated water.
The hurricane passed without destroying the tented camps in and around Port-au-Prince, which house about 1.3 million survivors of January's earthquake.
Aid workers say those living in the tent cities have better access to toilets and clean drinking water than the residents of some of the capital's long-standing slums, says the BBC's Laura Trevelyan in Port-au-Prince.
But if more cases are confirmed, the outbreak could threaten an estimated 2.5 to three million people in Port-au-Prince.
Cholera itself causes diarrhoea and vomiting, leading to severe dehydration. It can kill quickly but is treated easily through rehydration and antibiotics.