WASHINGTON (CNN) -- U.S. Secretary of State Colin Powell on Wednesday said the United States does not accept the results of Ukraine's presidential elections as legitimate, citing "credible reports of fraud and abuse."
The country's election commission has declared Prime Minister Viktor Yanukovych the winner, beating opposition leader Viktor Yushchenko by three percentage points.
Yushchenko said Wednesday evening that he did not recognize the official result and called for a countrywide "political strike."
Thousands of his supporters have protested for days in Kiev. (Opposition leader calls for nationwide political strike)
"If the Ukrainian government does not act immediately and responsibly there will be consequences for our relationship, for Ukraine's hopes for a Euro-Atlantic integration and for individuals responsible for perpetrating fraud," Powell said in a briefing at the State Department.
"We have been following developments very closely and are deeply disturbed by the extensive and credible reports of fraud in the election," he added. "We call for a full review of the conduct of the election and the tallying of election results."
Powell said it is "still not too late" for the Ukrainian government to remedy the situation. He warned that the world is watching and urged it to "seize the moment."
The secretary said he spoke with outgoing Ukrainian President Leonid Kuchma earlier Wednesday to urge him to take advantage of suggestions by the two candidates that there may be a way to resolve the controversy.
He also cautioned Kuchma not to use any kind of force against the thousands of pro-Yushchenko demonstrators in Kiev. He repeated that concern to Russian Foreign Minister Sergey Lavrov in a separate phone call, he said.
"We call on all sides to work to achieve a fair and just outcome without the use of force. We remind the Ukrainian authorities that they bear a special responsibility not to use or incite violence," the secretary said.
Powell said he also spoke with European Commission President Jose Barroso and High Representative Javier Solana, and said the United States supports an offer from Polish President Aleksander Kwasniewski to mediate in the situation.
"Tomorrow is the EU-Russian summit in Europe, and I'm confident this will be a subject of discussion between the EU leadership and the Russians," the secretary said.