The United Nations' torture investigator blamed Zimbabwe's president Robert Mugabe for banning him from entering the country today.

Manfred Nowak was forced to return to South Africa in what he described as a "serious diplomatic incident" that reflected a split in Zimbabwe's coalition government.

"There are certainly some parts of the government who do not want me to assess the current conditions of torture," he said, adding he thought Mugabe was behind the decision.

``There are strong indications that this was not just done by the Ministry of Foreign Affairs without at least the knowledge or instruction by President Mugabe," Mr Nowak said.

He had been due to meet Zimbabwe's Prime Minister Morgan Tsvangirai, long Mugabe's rival but now joined in a rocky coalition, at the start of a mission to investigate alleged attacks on Tsvangirai supporters by militants linked to Mugabe's ZANU-PF party.

Mr Tsvangirai, a long-time opposition leader, joined the government with Mugabe in February, but withdrew temporarily from Cabinet earlier this month after accusing ZANU-PF of human rights violations.