A U.N. investigator said he was barred from entering Zimbabwe for an eight-day trip to investigate alleged attacks against opposition party members.

Manfred Nowak said he learned that his mission was canceled after he arrived in Johannesburg, South Africa, en route to Zimbabwe, CNN reported

Zimbabwe said it canceled the mission so its leaders could consult with other southern African leaders on Zimbabwe's power-sharing agreement, the United Nations said. Nowak said he applauded consultation efforts, but said it wasn't a valid reason to cancel his investigative trip.

U.N. officials did not say whether Nowak's mission would be rescheduled, but stressed the allegations by opposition party Movement for Democratic Change and others about rights violations must be investigated.

"Allegations that MDC supporters and human rights defenders have been arrested, harassed and intimidated in recent days underscore the urgent need for an objective fact-finding mission by an independent U.N. expert," Nowak said.

The ban comes amid renewed tension between Prime Minister Morgan Tsvangirai and President Robert Mugabe, who reached a power-sharing deal after a disputed presidential election that prompted months of controversy. Tsvangirai's MDC party has accused Mugabe's Zanu-PF followers of human rights violations before, during and after the elections.