UN torture expert Manfred Nowak said his expulsion from Zimbabwe on Thursday showed that the country's unity government "does not function", in an interview with AFP.

"The government, as a unity government, does not function," he told AFP shortly after arriving in Johannesburg from Harare.

"It sheds a very negative light on the functioning of the government," he said.

He said he hoped that southern African officials who began meeting in Harare on Thursday could mediate an end to the crisis that has paralysed the unity government between long-ruling President Robert Mugabe and the new Prime Minister Morgan Tsvangirai.

But he later told a press conference: "I am afraid that it might as well escalate and this government falls apart."

Nowak was detained on his arrival late Wednesday in Harare, where he had planned to begin an eight-day mission to investigate alleged human rights abuses and arrests of political activists opposed to Mugabe.

He was expelled early Thursday -- despite having received an official invitation from Tsvangirai -- a decision the torture expert blamed on Mugabe's ZANU-PF party.

"The responsibility for the failure rests exclusively... with those ZANU-PF members of the government of Zimbabwe that actually produced the situation by denying me access to the country," he told reporters.

"It sheds a clear light as to where the real power is lying in this unity government," he added.