Abdullah Abdullah criticized the U.N. special envoy to the country, Kai Eide, accusing him of bias toward President Hamid Karzai and of not doing enough to uncover fraud in last month's unresolved elections, The Washington Post reported.
The U.N. and Afghan authorities are responsible for keeping the election running smoothly and investigating claims of fraud.
Abdullah raised questions about the U.N. after Eide's deputy, Peter Galbraith, was fired. Galbraith said he was dismissed over a dispute with Eide about how to handle the allegations of fraud. He also said Eide sided with Karzai -- a claim the U.N. denies.
Karzai won the election in an unofficial tally, but most of the allegations of fraud are against him and his supporters.
If the U.N. calls for a runoff, Abdullah would be Karzai's opponent. The investigation is expected to be complete by next week.