At least 21 protesters have been killed by security forces in Kiev following the breakdown of a truce agreed on Wednesday.
Witnesses have told the BBC that some died as a result of single gunshot wounds, typical of sniper fire.
Officials said that one policeman had died and that 67 police had been captured by protesters.
Meanwhile, three European Union foreign ministers have held five hours of talks with President Viktor Yanukovych.
For its part, the White House said it was "outraged by the images of Ukrainian security forces firing automatic weapons on their own people" and said the crisis "should be resolved by political means".
Eyewitnesses have told international news agencies that they have counted between 21 and 27 protesters' bodies after clashes in Kiev.
Video footage has emerged apparently showing snipers firing on demonstrators who had been trying to retake their protest camp in Independence Square.
The Kiev city administration said 67 people had now died in clashes since Tuesday.
Officials said more than 20 policemen had also been injured.
Witnesses reported live rounds, petrol bombs and water cannon being used at Independence Square during Thursday morning's clashes.
Some armed demonstrators were also reported to be firing towards security forces.
Gunshots pierced the windows of rooms at the Ukraine Hotel, which is serving as the base for all foreign media in Kiev, including the BBC and Sky News.
Earlier, several dozen protesters were using the lobby as a triage centre for the wounded, and a priest arrived, says the BBC's Kevin Bishop, at the scene.
Protesters - some of them armed - asked hotel guests for blankets to use as bandages.
A statement on the presidential website blames the opposition for starting the violence, saying the "calls for a truce and dialogue were nothing but a way of playing for time to mobilise and arm militants from Maidan [Independence Square]".
Opposition leaders called the violence "an act of provocation" by the authorities.
The foreign ministers of France, Poland and Germany conducted five hours of discussions with Mr Yanukovych, French Foreign Minister Laurent Fabius tweeted.
Polish Foreign Minister Radoslaw Sikorski tweeted that he would now stay in Kiev to meet opposition leaders to test a "proposed agreement", although it was not clear what the details of the agreement were.
Other EU foreign ministers, along with EU foreign policy chief Catherine Ashton, have convened at an emergency meeting in Brussels to discuss possible sanctions.
They could include a possible ban on sales of equipment that might be used for internal repression.
Separately, the head of the Kiev city administration resigned from Mr Yanukovych's Party of the Regions.
Thursday had been declared a day of mourning for those killed in clashes on Tuesday.
In other developments:
- Russia wants a "strong government" in Ukraine, Prime Minister Dmitry Medvedev said on Thursday, "so that people don't wipe their feet on the authorities like a doormat"
- Ukrainian alpine skier Bogdana Matsotska has withdrawn from the Sochi Winter Olympics in protest at the authorities' actions
- President Yanukovych's chief of staff has said if sanctions are imposed and the situation escalates, "there is a danger that the country could split into two parts," the Unian news agency reports
- Trains between Kiev and the western city of Lviv - one of the protesters' strongholds - have been suspended, the Interfax-Ukraine news agency reports. A railway spokeswoman said this was because of damage to the lines
- The UK Foreign Office has summoned the Ukrainian ambassador to ask him to call on his government to stop the violence
The EU has so far refrained from imposing sanctions on Ukraine, preferring to stress dialogue and compromise.
The US state department had already announced visa bans on 20 members of the Ukrainian government but did not provide any names.
The protests first erupted in November when President Yanukovych rejected a landmark association and trade deal with the EU in favour of closer ties with Russia.
Since then, the protests spread across Ukraine, with the main demand of snap presidential and parliamentary elections.
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