WASHINGTON (Sapa-AP) - The U.S. Senate is calling on Zimbabwean President Robert Mugabe to step aside and begin a peaceful transition to democratic rule.

In a non-binding resolution passed by voice vote late Tuesday, the Senate said the Zimbabwean Electoral Commission should immediately release the results of the March 29 presidential election, which Mugabe is widely believed to have lost.

Sen. John Kerry, a member of the Senate Foreign Relations Committee who lost the 2004 U.S. presidential election to George W. Bush, introduced the resolution.

He said "it was clear that the people of Zimbabwe have voted for a new beginning and it's imperative that President Mugabe accept their will and begin a timely and peaceful transition to democratic rule."

Alluding to South Africa, Kerry said key players in the region need to take the lead in bringing about the transition "but the United States, as the world's leading democracy, has a responsibility to speak out at this time of crisis."

Last week, the U.S. State Department backed off comments by its top Africa official asserting that Zimbabwe opposition leader Morgan Tsvangirai had won the presidential election.

"I'll just say that there is clearly a vote for change there and there are a lot of indications that he may well have won," said State Department spokesman Sean McCormack.

He would not go as far as Assistant Secretary of State for African Affairs Jendayi Frazer, who commented on the election during a visit to South Africa.

"We think in this situation we have a clear victor: Morgan Tsvangirai won, and perhaps outright," she said.

Nearly four weeks after the elections, the presidential results have not been published and a partial recount of the parliamentary elections continues.

The opposition has accused Mugabe of withholding the results while the 84-year-old autocrat who has ruled since independence from Britain in 1980 plots how to keep power.

At a news conference Tuesday President George W. Bush called on Zimbabwe's neighbours to step up pressure on Mugabe's government.

Bush said that "it's really incumbent on the nations in the neighbourhood to step up and lead."

He stopped short of saying that Mugabe had lost the election. But he added that it is clear that the country voted for change.

South African President Thabo Mbeki has been mediating the Zimbabwe crisis on behalf of the Southern African Development Community, known as SADC, using "quiet diplomacy" that has been criticized in many quarters but backed by many members of the U.N. Security Council.