Kenya's President Mwai Kibaki is seen in Nairobi in this Dec. 12, 2007 file photo

The U.S. State Department on Sunday congratulated Mwai Kibaki as the winner of Kenya's presidential election and called for calm in the African nation already disrupted by deadly riots.

But European Union election observers have questioned the credibility of the results.

The chief EU election monitor, Alexander Graf Lambsdorff said Kenya's electoral commission has failed to establish the credibility of the vote-counting process, because it has not addressed reported irregularities.

He said that, because of those irregularities, some doubt remains about the accuracy of the official results.

British Foreign Secretary David Miliband expressed "real concerns" over Kenya's election irregularities, calling on political leaders to address them "in a way that will bring the respect and support of the Kenyan people."

He urged all sides to settle their disputes peacefully through dialogue and the judicial appeals process.

The European Union team and other election observers initially declared Thursday's election to be free of fraud but withheld later judgment as delays developed during the vote-counting.

On Sunday, they EU said its observers were blocked from the count.

Some information for this report was provided by AFP, AP and Reuters.