The death toll from a cholera outbreak in Zimbabwe has risen to over 1,500 and case fatality rates are increasing, the United Nations said Saturday.

At least 1,518 people have now died of the disease and a total of 26,497 cases have been recorded since the start of the outbreak in August, the World Health Organization said. More than two-thirds of the deaths occurred in December alone.

Agency spokesman Paul Garwood said the cholera outbreak is still not under control and that neighbouring countries such as South Africa and Botswana, where the disease has also been reported, should boost their disease monitoring and preparedness.

But the UN agency is counselling against mass vaccination campaigns, which it says are unproven and provide a false sense of security.

The latest figures, provided by Zimbabwe's Ministry of Health on Dec. 25, indicate that new infections are occurring in all parts of the country despite massive international aid efforts to stop the disease.

The poor state of Zimbabwe's health infrastructure, combined with crumbling sanitation systems and the start of the rainy season, have contributed to the spread of the cholera, which is transmitted through contaminated water supplies.

"The epidemiological week ending 20 December saw over 5 000 new cases — an increase in the number of weekly cases relative to previous weeks — and an increase in deaths outside treatment/health centres,'' the agency said on its website.

The percentage of cholera patients dying from the disease has risen to 5.7 per cent from four per cent at the beginning of the month, the agency said. Normally only one per cent of patients die in large outbreaks, it said.