By Barnaby Phillips BBC southern Africa correspondent

Mediators from South Africa and Nigeria are reported to have flown to Zimbabwe in an attempt to broker negotiations between the Zanu-PF government and the opposition Movement for Democratic Change (MDC).

The MDC says President Robert Mugabe stole victory in last month's elections through widespread rigging.

Mugabe says that anyone who challenges his authority will be dealt with firmly

South Africa and Nigeria have been at the forefront of efforts to resolve Zimbabwe's crisis.

A South African government adviser told the BBC that a senior official in the ruling ANC, Kgalema Motlanthe, flew to Harare on Wednesday morning.

There he was expected to team up with a Nigerian envoy, Adebayo Adedeji. The two men have an unenviable task.

Difficult climate

A South African source said they would be concentrating on finding agreement between Zanu-PF and the MDC on ending violence and reviving the economy, as well as on land reform and looking at proposals for constitutional change.

But the atmosphere is not conducive to successful negotiations.

Neither the MDC nor the Zimbabwean Government is willing to give details of any possible talks, although one MDC official said they would be taking place in Vumba, in eastern Zimbabwe.

In public, the MDC has not retreated from its insistence that President Mugabe stand down, and that the elections be re-run.

The opposition says government-sponsored violence has increased since the elections.

President Mugabe, meanwhile, said at the weekend that anyone who does not accept his victory and who causes chaos will be dealt with very firmly.