Zimbabwean immigration officials barred the United Nations' torture investigator from entering their country and returned him to South Africa Thursday, an act he termed a "serious diplomatic incident" that reflects a split in the coalition government.

"There are certainly some parts of the government who do not want me to assess the current conditions of torture," Manfred Nowak angrily told reporters in Johannesburg upon arrival from Zimbabwe.

At a news conference later, he added he thought President Robert 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 of President Mugabe," Nowak said.

Rocky coalition

Nowak said he had a meeting scheduled Thursday with Zim-babwe'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.

Tsvangirai, a longtime 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.