An outraged U.N. torture investigator said Thursday he believes Zimbabwe's president may be the reason he was blocked from the country, a move that could further isolate the struggling nation and its fragile coalition government.

Manfred Nowak had planned to investigate alleged attacks on Prime Minister Morgan Tsvangirai's supporters by militants linked to President Robert Mugabe's ZANU-PF party. Mugabe, who has been in power for nearly three decades, is accused of trampling on human rights and democracy, and holding the international community in contempt.

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

"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," Nowak said later at a news conference.

Ephraim Masawi, a spokesman for Mugabe's ZANU-PF party, said angrily that allegations Mugabe was involved in barring Nowak were "not true."

Nowak said he had a meeting scheduled Thursday with Tsvangirai, even though other Zimbabwean officials had told him he was not welcome. Tsvangirai, a longtime opposition leader, joined the government with Mugabe in February, but withdrew temporarily from Cabinet earlier this month after accusing Mugabe's party of human rights violations.

Nowak called his treatment a "serious diplomatic incident" as well as alarming evidence of the split in the coalition that Tsvangirai has called the only way to rescue Zimbabwe from economic ruin and violent political impasse. Nowak said the coalition could fail, which would mean new balloting in a country with a history of election violence blamed on Mugabe's supporters.

Jean Ziegler, who advises the United Nations on human rights issues, said the U.N. general-secretary could — and should — ask U.N. aid and development agencies to curtail cooperation with Zimbabwe in response to the "scandalous" treatment of Nowak.

Ziegler, who has previously served as a U.N. special investigator, known as a rapporteur, said failing to respond strongly to what happened to Nowak could inspire "any other lawless state" to behave similarly.

Amnesty International's Zimbabwe researcher, Simeon Mawanza, said Nowak's barring reflected an "increased level of desperation among those forces who are opposed to the unity government."

Sipho Mthathi, a Human Rights Watch expert on Zimbabwe, called on Zimbabwe's neighbors to tighten oversight of the coalition agreement they sponsored, and for international sanctions to be broadened. Western countries have slapped travel bans and asset freezes on Mugabe and his top aides, and Mthathi said African countries should follow suit.

Human Rights Watch also called Thursday for Zimbabwe to be suspended from the Kimberley Process, which would make it harder for Zimbabwean diamonds to reach international markets.

Human Rights Watch says Zimbabwean soldiers are smuggling diamonds, and killing and beating civilians to consolidate their hold on a gem-rich area in the east of their country. A Kimberley Process suspension would, like the West's targeted sanctions, spare ordinary Zimbabweans because only the powerful, ZANU-PF-connected were benefiting from the diamond trade, Mthathi said.

Nowak was formally invited to investigate torture allegations Oct. 1 — before Tsvangirai withdrew from the Cabinet — by Justice minister Patrick Chinamasa, a ZANU-PF leader.

The U.N. envoy received word from the foreign ministry, also controlled by ZANU-PF, that he should not come only after he had flown from Austria to South Africa and was hours from reaching neighboring Zimbabwe.

Still, Nowak flew to Zimbabwe Wednesday, citing a renewed invitation from Tsvangirai. When he arrived, airport immigration officials told him the foreign ministry had not cleared his meeting with the prime minister, he said. He spent the night in the airport.

"I have never been treated as rudely by any government as the government of Zimbabwe," Nowak snapped.

Nowak said he contacted Tsvangirai's office from the airport, which sent a high-level delegation to fetch him. The delegation was barred by airport security, and was even told Nowak was not at the airport, the U.N. envoy said.

Tsvangirai's spokesman, James Maridadi, said Thursday that Nowak's trip had been cleared and that he could not immediately say why he had been barred.

"We are surprised that he was detained last night at Harare International Airport," Maridadi said.

Joey Bimha, the top civil servant in the ZANU-PF-controlled foreign ministry, said Nowak had been told he could not come because officials were engaged with Tsvangirai's temporary withdrawal from the Cabinet. A trio of foreign ministers from neighboring countries was holding talks with the factions Thursday and Friday in an attempt to end the impasse.

"We had no option but to send (Nowak) back because we had informed him that his services were no longer needed here," Bimha said.