US President George W. Bush joined calls for Robert Mugabe to step down yesterday, 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 Mr 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 people in a worst-case scenario.

"It is time for Robert Mugabe to go," Mr 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."

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