The United States said Friday it believes Nigeria's presidential elections were "seriously flawed" and urged all parties to work out a peaceful solution.

In a special statement, State Department deputy spokesman Tom Casey said there have been credible reports of fraud and vote rigging during last Saturday's elections.

He said the United States is urging Nigerians to seek a remedy through a constitutional and civilian-controlled process.

Nigeria's electoral commission declared ruling party candidate Umaru Yar'Adua the winner even though Nigerian and international observers found numerous problems with the vote.

On Friday, Nigeria's main opposition party said it will go to court to challenge the official results.

Speaking to reporters, the chairman  of the All Nigeria People's Party also vowed not to recognize or cooperate with any government formed as a result of last week's vote.

A coalition of 20 opposition groups have called for new elections.