The United Nations' special rapporteur on torture who was barred from entering Zimbabwe to meet Prime Minister Morgan Tsvangirai left Harare Thursday for South Africa, his office said.

"They are on their way back to Johannesburg and they will decide their next step," Claudia de la Fuente told AFP speaking for the office of special rapporteur Manfred Nowak.

Nowak and his team were due to resume an eight-day fact-finding mission to investigate alleged human rights abuses and arrests targeting supporters of Tsvangirai's Movement for Democratic Change.

He was denied entry by immigration officers, shortly after landing in Harare on Wednesday evening.

The state-run Herald newspaper said Nowak had tried to "gatecrash into the country" as foreign ministers from the regional Southern African Development Community (SADC) arrived for a Thursday meeting to assess the unity pact's progress.

"Government had already communicated to him that he would have to visit on a later date because the state was presently hosting foreign ministers from the SADC organ on Politics, Defence and Security," the Herald newspaper said.

"We informed Geneva and the UN office here that he should postpone his visit," Foreign Affairs Secretary Joey Bimha told the newspaper.