JOHANNESBURG, South Africa - Britain and the United States increased pressure on Zimbabwean President Robert Mugabe to step down, accusing him of presiding over the country's economic collapse blamed for a cholera outbreak that has killed more than 1,000.

But the calls are more likely to harden the stance of Mugabe, who does not want to be seen as bowing to demands from white Westerners.

Britain's Africa Minister Mark Malloch Brown said Monday that Mugabe must retire for a power-sharing government to succeed in the former British colony facing a mounting economic and humanitarian crisis.

He told BBC radio that Mugabe was incapable of making good on a deal reached in September to govern alongside opposition leader Morgan Tsvangirai.

"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."

The remarks came a day after the top U.S. diplomat for Africa, Jendayi Frazer, said Washington can no longer support a Zimbabwean deal that leaves Mugabe in office as president. Also stepping up pressure, the Roman Catholic Bishops Conference of Southern Africa called for Africans and especially regional giant South Africa "to isolate Mugabe completely."

But Mugabe, once considered a hero among African freedom fighters, has shrugged off such criticism, drawing many Africans to his side with claims he is fighting a Western imperialist plot.

"The only likelihood is that they (African leaders) will harden in their stand against so-called Western imperialism," said John Makumbe, a political science professor in Zimbabwe. "I think (Mugabe) actually enjoys all that pressure and sees it as giving him the limelight."

African leaders are also wary of being seen as simply following the U.S. and now the British lead. Frazer on Sunday acknowledged that stepping up the pressure against Mugabe could backfire. But she said it was a risk worth taking, because "at some point we have to say what we really believe."

Mugabe, 84, has ruled the country since its 1980 independence from Britain and refused to leave office following disputed elections in March.

He has faced renewed criticism amid a humanitarian crisis that has pushed millions of Zimbabweans to the point of starvation and spawned a cholera epidemic that has killed more than 1,000 people since August.

President George W. Bush, British Prime Minister Gordon Brown and French President Nicolas Sarkozy all have called for Mugabe to step down.

Those few Africans who have spoken out against him have been denounced as "lackeys" obeying the orders of white masters. The Catholic bishops said it was time for that to stop.

"Some African leaders, to their shame, have felt it necessary to stand in solidarity with Mugabe against the supposed machinations of former colonial and present imperial powers; it is time for them to redirect their solidarity towards the needs of the suffering people of this once-thriving country," they said Sunday.

Britain and the United States keep urging African governments, especially those in southern Africa, to take concerted action against Mugabe. But there is little they can do to put pressure on the Africans.

"I think this is a hardening of rhetoric by the U.S. and the U.K., but I don't think that is reflected in the thinking of the Southern African Development Community, or the African Union," said Alex Vines, head of the Africa program at London's Chatham House think tank.

Meanwhile, he thinks "the impasse will continue."

On Friday, an ever-defiant Mugabe declared that "Zimbabwe is mine," saying only Zimbabweans can remove him from power and that no African nation is brave enough to wrest it from him.

"The real pressure will have to come from within Zimbabwe, through civic action, through the military rioting, work boycotts by teachers, the nurses and the doctors to keep the hospitals and schools closed," Makumbe said.

Zimbabwe, once the region's breadbasket, has seen its agricultural sector collapse under Mugabe. There are chronic shortages of everything including food, medicine, fuel and cash.

Critics blame Mugabe's policies for the nation's ruin. Mugabe blames Western sanctions, though the European Union and U.S. sanctions are targeted only at Mugabe and dozens of his clique with frozen bank accounts and travel bans.

This month, soldiers rioted in downtown Harare when they could not withdraw their salaries from banks that ran out of cash; all the main hospitals in Harare are closed, because staff have not been paid or because they have no medication.

The bishops called for South Africa's President Kgalema Motlanthe "to stop immediately all collusion with Mugabe and to cut off any lifeblood that South Africa is offering him." Specifically, they suggested cutting fuel and electricity supplies to landlocked Zimbabwe.

Last month, Botswana's Foreign Minister Phandu Skelemani called for African nations to close their borders with Zimbabwe, saying it would bring Mugabe down in just a week or two.

But South Africa maintains the answer for Zimbabwe is power-sharing, not ousting Mugabe.

Malloch Brown suggested that Mugabe might be moved by a promise of immunity from international prosecution for alleged crimes against humanity.

"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.