Manfred Nowak, the U.N. special rapporteur on torture, said he had been invited by the prime minister, Morgan Tsvangirai, and wanted to investigate reports of rising violence and intimidation. He believes that the president, Robert Mugabe, may have given the order to deny him entry.

"I have never been treated as rudely by any government as the government of Zimbabwe," Nowak said after arriving back in Johannesburg, South Africa. "This mission has now failed. A lot of money was wasted because of the unacceptable behavior of the government."

The "serious diplomatic incident" happened days after Tsvangirai's Movement for Democratic Change disengaged from Mugabe's Zanu-PF party, citing human rights violations and persistent breaches of their power-sharing agreement.

A delegation of ministers from southern Africa began talks with the parties Wednesday in an attempt to prevent the unity government from collapsing.