The U.S. embassy in Tanzania said it is "gravely alarmed" at the recent annulment of the Zanzibar presidential election.
Semi-autonomous Zanzibar has its own government, but its votes count in Tanzania's national presidential election, which was held October 25.
The embassy said late Wednesday that the Zanzibar action nullified "an orderly and peaceful election," as observed by officials from the U.S., the European Union, the Commonwealth, and the Southern Africa Development Community. In a statement, U.S. officials called for the action to be withdrawn and urged "all parties to maintain a commitment to a transparent and peaceful democratic process."
Zanzibar's election commission Wednesday said the results were nullified because of "gross violations" in the voting process. Commission chairman Jecha Salim said there is a need to hold fresh elections in the archipelago.
The nullification is also likely to raise tension on Zanzibar, where a local opposition party had claimed victory on Monday before results were announced.
Tanzania's recent elections for president, parliament and local seats were expected to be the closest in the nation's history. President Jakaya Kikwete is stepping down after completing two five-year terms.
The ruling CCM party is facing its first major challenge in decades. Several Cabinet ministers have lost their parliamentary seats.
The CCM has faced increasing pressure to speed up the country's development and deal with a persistently high poverty rate.
Commission chairman Judge Damian Lubuva expressed confidence Sunday the electoral process will be judged free and fair, despite reported problems at some polling stations in the country.
Appearing on a live VOA Swahili broadcast, Judge Lubuva admitted there were some discrepancies at some polling stations, including lack of voting materials, but he said there will be no repeat voting exercises, except in constituencies where the process was postponed.