Cholera cases among cyclone survivors in Mozambique have jumped to 271, authorities said, a figure that has nearly doubled from the previous day.
Portuguese news agency Lusa cited national health director Ussein Isse, who declared the outbreak of the acute diarrhoeal disease on Wednesday with just five cases.
So far no cholera deaths have been confirmed, the report said.
Cases of the diesease have been discovered in the port city of Beira, whose half-million residents are at particular risk, especially those in crowded, poor neighbourhoods.
Doctors Without Borders said it is seeing 200 probable cholera cases a day in the city, where relief workers are hurrying to restore the damaged water system and bring in additional medical assistance.
The World Health Organisation has said 900,000 cholera vaccine doses are expected to arrive on Monday, with a vaccination campaign starting later in the week.
Cholera is spread by contaminated food and water and can kill within hours if not treated.
The disease is a major concern for the hundreds of thousands of cyclone survivors in the southern African nation now living in squalid conditions in camps, schools or damaged homes.
Some drink from contaminated wells or filthy, stagnant water.
As health responders stress the need for better disease surveillance, the United Nations' deputy humanitarian co-ordinator in Mozambique, Sebastian Rhodes Stampa, has said all cases of diarrhoea are being treated as though they are cholera.
Cholera is endemic to the region, and "it breaks out fast and it travels extremely fast", he told reporters.
Doctors Without Borders has said other suspected cholera cases have been reported outside Beira in the badly hit areas of Buzi, Tica and Nhamathanda, but the chance of spread in rural areas is smaller because people are more dispersed.
Mozambican officials have said Cyclone Idai destroyed more than 50 health centres in the region, complicating response efforts.
The cyclone also killed at least 259 people in Zimbabwe and 56 in Malawi.
The UN has said 1.8 million people need urgent help across the sodden, largely rural region.