The UN has urged African leaders at a key summit in Egypt to try to negotiate a solution to the crisis in Zimbabwe, reports BBC.
UN Deputy Secretary General Asha-Rose Migiro said this was the "moment of truth" for the African Union leaders.
President Robert Mugabe is attending the meeting. He was sworn in Sunday after a victory that observers said had been undermined by pre-poll violence.
South Africa has now urged Mr Mugabe to hold talks with the opposition towards forming a transitional government.
Mr Mugabe claimed a landslide victory as the sole candidate after the leader of the opposition Movement for Democratic Change (MDC), Morgan Tsvangirai, withdrew.
Ms Migiro told the leaders of the 53-nation bloc: "This is a moment of truth for regional leaders... the secretary-general urges your excellencies to mobilise support for a negotiated solution."
She added: "This is the single greatest challenge to regional stability in southern Africa."
Ms Migiro again expressed UN regret that the election had been allowed to go ahead despite the violence.
In his welcoming speech, host President Hosni Mubarak said bolstering peace and security was "essential for resolving disputes and conflicts in the continent".
The AU has a rule not to accept leaders who have not been democratically elected - but observers say it is unlikely to take such strong action against Mr Mugabe so quickly.
A draft resolution written by African foreign ministers during talks ahead of the summit did not criticise the elections or Mr Mugabe, but condemned violence in general terms and called for dialogue.
Independent observers have criticised the poll.
The AU's own monitors said on Monday: "The vote fell short of the African Union's standards of democratic elections."
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South Africa's Thabo Mbeki is the key mediator. He has not criticised Robert Mugabe, despite pressure from the ruling ANC.
Zambian President Levy Mwanawasa has taken the toughest line. He's called Zimbabwe a "regional embarrassment".
