HARARE, Zimbabwe - The UN secretary-general and the leaders of Britain and the African Union urged Zimbabwe on Wednesday to ensure that the outcome of the country's contested presidential election is not rigged.

"A stolen election would not be a democratic election at all," British Prime Minister Gordon Brown told a high-level meeting of the UN Security Council on African peace and security issues.

After 18 days, Zimbabwean electoral officials have yet to say whether President Robert Mugabe or opposition leader Morgan Tsvangirai won the March 29 presidential election.

Brown, whose country was the former colonial ruler, was the most outspoken, saying "no one thinks, having seen the results at polling stations, that president Mugabe has won this election."

Tsvangirai says he won the election outright and has accused Mugabe of withholding the results to maintain his 28-year grip on power. Independent tallies show Tsvangirai won, but not by enough to prevent a runoff.

UN Secretary-General Ban Ki-moon said he was deeply concerned at the failure to release the election results.

"If there is a second round of elections, they must be conducted in a fair and transparent manner, with international observers," Ban said. "The United Nations stands ready to provide assistance in this regard."

The leaders of the Southern African Development Community held a summit in Zambia that ended before dawn Sunday with a weak declaration that failed to criticize the absent Mugabe.

The declaration called for the quick verification of election results in the presence of the candidates or their agents, and urged "all parties to accept the results when they are announced."

South Africa, which holds the Security Council presidency this month, organized the meeting to focus on cooperation between the United Nations and the African Union, and African crises on the council's agenda.

South Africa's UN Ambassador Dumisani Kumalo said Zimbabwe was not on the agenda because it is being dealt with by SADC.

In his opening remarks, South African President Thabo Mbeki, who chaired the council meeting, made no mention of Zimbabwe, focusing instead on UN-AU relations and the issue of funding regional peacekeeping operations.

Mbeki, the chief mediator on Zimbabwe, has pursued a policy of "quiet diplomacy," which some critics say simply allowed Mugabe to continue his autocratic rule. After meeting with Mugabe in Zimbabwe on Saturday, just before the SADC summit, Mbeki said "there is no crisis."