Zimbabwe's President Robert Mugabe "has lost all sense of reality" and has "become a mad dictator," according to a United Nation's human rights expert.

Jean Zigler told Swiss Radio on Monday that, "the horror in Zimbabwe today is absolutely intolerable."

The comments from the Swiss sociologist, reported by Reuters, reflected the despair over Zimbabwe on the rights council.

Four other U.N. rights experts said Zimbabwe could not control a cholera epidemic that has killed more than 1,100 people, and that Mugabe's "violations of civil and political rights" made it difficult to get a united response to the crisis.

"Europe should call on South Africa to abandon its support for this mad dictator so as to open the way to a government of the opposition, which won the elections," Zigler said.

Meanwhile, Britain's minister for Africa said Monday that Mugabe must step down because he is incapable of making good on a deal to share power with opposition leader Morgan Tsvangirai.

His comments follow remarks made over the weekend by the top U.S. envoy for Africa, who said Washington can no longer support a Zimbabwean deal that leaves Mugabe in office as president.

"Power-sharing isn't dead but Mugabe has become an absolute impossible obstacle to achieving it," Malloch Brown said. "He's so distrusted by all sides that I think the Americans are absolutely right — he's going to have to step aside."

Mugabe, 84, has ruled the country since its 1980 independence from Britain and refused to leave office following disputed elections in March. A power-sharing agreement with the opposition was struck in September, but has been stalled by an impasse over how to divide Cabinet posts.

Mugabe has faced renewed criticism amid a humanitarian crisis that has pushed thousands of Zimbabweans to the point of starvation and left more than 1,000 people dead from cholera since August. President George W. Bush, British Prime Minister Gordon Brown and French President Nicolas Sarkozy have called for Mugabe to step down.

"The final death throes of these kinds of situations seem terribly slow and grim and unnecessary," Malloch Brown said.

Jendayi Frazer, the U.S. assistant secretary of state for African affairs, said she had become convinced Mugabe is incapable of sharing power. If Mugabe's neighbors were to unite and "go to Mugabe and tell him to go, I do think he would go," she said Sunday.

Malloch Brown said he doubted that Mugabe would willingly leave his office.

"In this era of the international criminal court, it's very hard for any particular country to offer that guarantee," he said. "I think that if President Mugabe was to come to the U.K. and the U.S. or other third parties — African neighbors — and say 'I'll go if I can be offered a quiet retirement,' I expect people would look at what's possible," he told the BBC.

The U.S. Office of Foreign Assets Control last month blacklisted 21 businesses tied to Mugabe — 18 of which are based in Britain, or British territories overseas — but Malloch Brown said the firms don't yet have restrictions on their activities in the U.K.

"Where the U.S. moves, we try to stay as closely in line with them as possible. Our procedures, because they're done through Europe, are slower," Malloch Brown said. "It is the case that totally owned Zimbabwean subsidiaries of companies registered here in the U.K. do still have freedom to operate, but we do not believe that there are sanction-busting activities being run out of the U.K."

The Associated Press and Reuters contributed to this report.