South African health officials have described Zimbabwe's cholera epidemic as "unprecedented". But as Thami Mseleku, director general of the South African health ministry, was describing the "massive magnitude" of the epidemic, Zimbabwean President Robert Mugabe said the outbreak was over. The claim that was swiftly rejected by Britian's Africa Minister Mark Malloch-Brown. The United Nations raised the death toll from the waterborne epidemic in Zimbabwe to 783 on Thursday, with a further 16,403 people infected. Cholera has spread rapidly in Zimbabwe because of the country's crumbling health care system and lack of clean water - people have been forced to drink from contaminated wells and streams. Last week, Zimbabwe declared a health emergency because of the disease and the collapse of its health services. Aid agencies have also warned about coming rains further spreading cholera among a population already weakened by disease and hunger. South Africa has been caring for scores of Zimbabwean cholera victims who have crossed the border seeking help. Almost 500 cholera cases have been detected in South Africa, nine of whom died. South Africa has withheld 300 million rands (£20m) in aid for Zimbabwe but otherwise has proved reluctant to use its huge economic and political muscle against its neighbour. Earlier in the week, Mr Mugabe faced calls from Western leaders to step down. US President George W Bush said: "It is time for Robert Mugabe to go. Across the continent, African voices are bravely speaking out to say now is the time for him to step down." Prime Minister Gordon Brown went a step further, saying Mr Mugabe's "blood-stained regime" was responsible for the cholera epidemic. The world must tell Mr Mugabe "enough is enough," he said. But the African Union made clear it did not back calls for much tougher action. The crisis in Zimbabwe has been exacerbated by political deadlock between Mr Mugabe and opposition leader Morgan Tsvangirai over the implementation of a power-sharing deal brokered by South Africa. Basic foodstuffs are running out, prices of goods are doubling every 24 hours, and the 100 million Zimbabwean dollar a week limit for bank withdrawals buys only three loaves of bread in the once relatively prosperous country. Meanwhile, a British Red Cross appeal to tackle the cholera epidemic and chronic food shortages in Zimbabwe is under way amid speculation that the crisis could prompt people from the country to head for the UK. Home Secretary Jacqui Smith has reportedly warned the Cabinet of a possible mass influx of Zimbabweans as a result of the outbreak.