US President George W Bush joined calls for Robert Mugabe to step down today, but the African Union rejected tougher action against Zimbabwe's president and said only dialogue could solve its crisis.
The death toll neared 600 from a cholera epidemic which Mugabe's government accuses Western powers of exploiting to try to force his departure. The World Health Organisation said cholera could affect as many as 60,000 in a worst-case scenario.
"It is time for Robert Mugabe to go," Bush said in a statement in Washington.
"Across the continent, African voices are bravely speaking out to say now is the time for him to step down."
But the African Union earlier made clear it did not back calls for much tougher action.
"Only dialogue between the Zimbabwean parties, supported by the AU and other regional actors, can restore peace and stability to that country," said Salva Rweyemamu, spokesman for AU chairman and Tanzanian President Jakaya Kikwete.
Rweyemamu said sending peacekeeping troops or removing Mugabe by force, as proposed by prominent figures including Kenyan Prime Minister Raila Odinga and Nobel peace laureate and South African Archbishop Desmond Tutu, were not options.
"We have a serious humanitarian crisis in Zimbabwe. We have cholera. Do they think that we can eradicate cholera with guns?"
South Africa will oppose any move to send troops to Zimbabwe, a senior government official there said.
Mugabe and opposition leader Morgan Tsvangirai reached a power-sharing deal brokered by regional mediator Thabo Mbeki, South Africa's former president, in September. But they are deadlocked over how to implement it.
The spreading cholera, coupled with chronic food shortages, has highlighted the economic collapse of the southern African country, once relatively prosperous. Basic foodstuffs are running out and prices of goods have been doubling every day.