U.N. Secretary-General Ban Ki-moon said on Monday he was talking to African states about how the world body could help ensure an election run-off in Zimbabwe is credible and voiced concern at growing violence.

Zimbabwe's opposition MDC has yet to say whether its leader Morgan Tsvangirai would contest a second round against President Robert Mugabe, but has said one condition for doing so might be a U.N.-led observer mission.

The opposition rejects results showing Tsvangirai failed to beat Mugabe by a big enough margin in the March 29 vote to avoid a run-off and accuses his supporters of a campaign of violence.

But to pull out now would mean Mugabe automatically keeps a 28-year hold on power in his ruined country.

Ban told reporters at U.N. headquarters he would discuss a possible U.N. role in Zimbabwe with Tanzanian President Jakaya Kikwete, who chairs the African Union, and Zambian President Levy Mwanawasa, who leads southern Africa's regional bloc.

"I have been constantly following and monitoring the situation and discussing how the United Nations and the African Union together can help the Zimbabwean situation reach a very harmonious and credible way," he said in New York.

He said U.N. monitors and sending a U.N. envoy to Zimbabwe were both options.

"These are some ideas which I am going to discuss, what would be the practical possible ways at this time," he said.

Regional countries might be ready to press Mugabe to accept U.N. monitors. In the first round, observers from his Western critics were barred and the main monitoring group was from the Southern African Development Community.

The regional grouping has faced criticism for a cautious diplomatic approach to Zimbabwe in recent years, but showed signs of impatience with Mugabe as a month-long delay to the election results fanned fears of widespread bloodshed.

VIOLENCE

The opposition, Western governments and human rights groups accuse Mugabe of unleashing militias to scare Zimbabweans into backing him in the run-off. The government denies the allegation and says the MDC is behind political violence.

The MDC says at least 20 of its supporters have been killed and more than 1,000 homes burnt or destroyed since the election.

"I am deeply concerned at reports of rising levels of violence and intimidation within Zimbabwe," said Ban.

A union said teachers, who make up the bulk of polling officers, have been targets of a violent campaign. The Progressive Teachers' Union of Zimbabwe (PTUZ) said in a statement over 2,740 teachers had been targeted.

Tendai Chikowore, head of Zimbabwe's main teachers' union, the Zimbabwe Teachers' Association (ZIMTA), said more and more teachers were victims of the campaign.

No date has been set for a run-off yet. By law, the Zimbabwe Electoral Commission (ZEC) is supposed to set the date within 21 days of the result. But it has the power to extend the period and political observers believe a 40-day period is likely.

Utoile Silaigwana, ZEC's deputy chief elections officer, would not be drawn on whether it planned to extend the date.

"The commission will soon meet over this matter," he said.

MDC officials said party leaders were hammering out conditions they would demand for Tsvangirai's participation in a second round - including a strong international observer mission and the speedy release of results.

The opposition says the month-long delay in announcing the last result allowed the outcome to be rigged. The official result showed Tsvangirai won 47.9 percent of the vote to Mugabe's 43.2 percent.