President Leonid Kuchma called for negotiations among all sides in Ukraine's spiraling political crisis Tuesday, hours after the opposition leader declared himself the winner of a disputed presidential election and took a symbolic oath of office.

A top opposition figure accepted the outgoing president's proposal, the Interfax news agency reported.

"We now have decided to give the possibility to Kuchma to form proposals for talks," Yuliya Tymoshenko said, according to Interfax. It was not immediately clear when the talks might take place.

The startling development in the three-day standoff came as the Bush administration urged the Ukrainian government not to certify results of Sunday's runoff election results that showed Prime Minister Viktor Yanukovych, backed by Kuchma, defeating the Western-leaning Viktor Yushchenko.

"The United States is deeply concerned by extensive and credible indications of fraud committed in the Ukrainian presidential election," White House spokeswoman Claire Buchan said.

But Russian President Vladimir Putin, who supported Yanukovych in the campaign, criticized Western assessments of the vote as flawed, stressing that the results were not yet official. On a state visit to Portugal, he called for calm and respect for the law in this former Soviet republic.

Kuchma's statement was read on state television as tens of thousands of Yushchenko's supporters stood their ground in the tense, freezing streets of Kiev for a third straight day. They have vowed to stay until officials nullify the results of an election they say was rigged.

In his statement, Kuchma referred to the demonstrations as "political farce ... (which is) extremely dangerous and may lead to unforeseeable consequences."

Source: Agencies