Western countries, including Canada, warned Ukraine's government they may impose sanctions or other punitive measures as increasingly violent anti-government protests turned deadly.

Canada has called Ukraine's ambassador on the carpet, The Globe and Mail has learned, as Ottawa and Washington appear to be consulting the European Union on whether to impose sanctions that would target those in power in Kiev.

At least two protesters were shot and killed Wednesday during clashes with police in Ukraine's capital, the first protest-related fatalities in a political crisis sparked by Ukrainian President Viktor Yanukovych's decision to balk at closer ties with the European Union and instead align himself more closely with Vladimir Putin's Russia. The latest confrontations were sparked by new laws limiting the right to protest.

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As a central Kiev street burned, several thousand protesters fought with police who violently beat and shot at those assembled, a throng that included volunteer medics and journalists. Security forces were equipped with an armoured personnel carrier and the power to shut streets and fire water cannons in the freezing weather.

Ukrainian opposition leaders, three of whom met with the President, issued an ultimatum to Mr. Yanukovych: Call early elections or face more public anger. One leader, boxer-turned-politician Vitali Klitschko, threatened to lead protesters "on the attack."

Mr. Klitschko told the thousands of protesters gathered on Kiev's Independence Square that during three hours of talks, Mr. Yanukovych had given no clear response to the opposition's demands.

"[The police] are preparing to clear us out of the Independence Square," Mr. Klitschko declared. "We must do all we can to stop them."

He urged people to stay overnight and defend the square and drew a roar of support from protesters when he declared: "If I have to go [on to the streets] under bullets, I shall go there under bullets."

Canada's Foreign Affairs Minister John Baird called the killings of protesters disturbing and said the situation in Ukraine "obviously requires condemnation and response" from the international community.

"All options are on table. I think we need to be very clear that we can't remain silent, but I think it will be absolutely essential for us to consult with like-minded [partners], particularly the United States and European Union,"

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The United States said it had revoked American visas for several Ukrainian it holds responsible for violence in the Eastern European country and said it was considering "additional steps in response to the use of violence."

José Manuel Barroso, who helms the European Union's executive branch, blamed the Ukrainian government for not taking steps to "de-escalate this crisis" and warned the union would be considering "possible actions."

Canada summoned Ukraine Ambassador Vadym Prystaiko to the Department of Foreign Affairs Wednesday over the legislative crackdown on dissent and the police attacks on protesters, sources said.

The Canadian government told Mr. Prystaiko that Canada deplores the killing of protesters and called on Ukraine to hold an independent inquiry into their deaths. Ottawa told the ambassador that it is "actively considering" sanctions along with other options but has not reached a decision, sources added.

Ukraine's deepening crisis began in late November after Mr. Yanukovych backed off signing a co-operation deal with the 28-member European Union. In mid-December, the Kiev leader also accepted assistance from Russia including a major discount on Russian natural gas and Moscow's offer to buy billions of dollars of Ukrainian government bonds.

With reports from Reuters, Associated Press and Agence France-Presse