Harare - The United Nations's special rapporteur on torture, Manfred Nowak, was flying back to South Africa on Thursday after being barred from entering Zimbabwe where he was to investigate allegations of torture by the security forces.

A major diplomatic spat between Zimbabwe and the UN now looms after Nowak was detained by immigration officials at Harare airport on Wednesday evening and made take the first flight out of the country back to Johannesburg on Thursday.

"He's on the flight back to Johannesburg right now," a source close to his office told the German Press Agency dpa. The flight was due to arrive in Johannesburg around 9 am (0700 GMT).

Nowak reacted angrily to his detention by immigration officials on his arrival in Harare on the grounds that he had "no security clearance."

"I should say I have never been treated in any other country in this way," Nowak, who became special rapporteur in 2004, said by phone from the airport. "This is a major diplomatic incident."

The incident, which coincides with a sudden worsening of relations between coalition partners President Robert Mugabe and Prime Minister Morgan Tsvangirai, is all the more remarkable given that Nowak had actually been invited to Zimbabwe by the government.

Mugabe's justice minister, Patrick Chinamasa, extended the invitation in February after the reported widespread incidence of torture by Mugabe's security forces.

But in a last-minute U-turn, the foreign ministry, which is also controlled by Mugabe's Zanu-PF, withdrew the invitation and asked Nowak to postpone.

The government gave as its reason a one-day visit Thursday by three foreign ministers from the Southern African Development Community, the 15-nation regional bloc, which is attempting to resolve the stalemate in the unity government.

The eight-month-old transitional government formed by Zanu-PF and Tsvangirai's Movement for Democratic Change (MDC) is in crisis after the MDC began boycotting cabinet meetings two weeks ago. The party accuses Zanu-PF of failing to keep its side of the power-sharing deal by blocking key reforms.

Nowak has questioned the government's stated reason for its snub, noting that the SADC meeting is a one-day affair and that he was planning to stay for a week.

On Thursday he had been due to meet with Tsvangirai, whose office had given him a letter of invitation that was supposed to ease his passage through immigration, but evidently failed.

The Office of the High Commissioner for Human Rights in Geneva, to which Nowak is attached, said his visit was crucial given reports of a renewed campaign of intimidation against Mugabe critics.

"Recent allegations that MDC supporters and human rights defenders have been arrested, harassed and intimidated during the past few days, highlight the urgency of objective fact-finding by an independent UN expert at this crucial stage," the OHCHR said.