European Union nations have agreed sanctions on Ukraine as a second day of bloody clashes on the streets of the country's capital Kiev left dozens of anti-government protesters dead.
Unconfirmed reports suggested that at least 70 died and hundreds more were injured as government snipers fired at demonstrators after firebombs were hurled at police lines.
EU foreign ministers meeting in Brussels agreed to impose sanctions on officials held responsible for the violence, including a travel ban and asset freeze on close allies of Ukraine's president Viktor Yanukovych.
The White House said the US was outraged by scenes of Ukrainian security forces firing automatic guns on protesters and urged Mr Yanukovych to withdraw forces immediately from central Kiev.
Prime Minister David Cameron embarked on a round of telephone diplomacy, agreeing with German chancellor Angela Merkel on the "particular responsibility" of Mr Yanukovych to bring an end to the bloodshed. Mr Cameron is expected to speak by phone with other European leaders over the next 24 hours and will discuss the crisis with Dutch PM Mark Rutte when he visits the UK tomorrow.
Speaking after Mr Cameron's call to Mrs Merkel, a Downing Street spokesman said: "They are both extremely concerned by the spiralling violence. They agreed that the immediate priority must be to stop the violence and that President Yanukovych has a particular responsibility to help de-escalate the situation by pulling back government forces.
"They discussed how they could work together and with international partners to foster a solution to the crisis. They agreed to do all they can to secure support from all sides in Ukraine for a possible roadmap, proposed by European foreign ministers in Kiev earlier today, which could lead to a peaceful solution to the crisis."
President Barack Obama's press secretary Jay Carney said in a statement that Ukraine should respect the right of protest and that protesters must be peaceful. The statement called for dialogue to reduce tensions and address grievances expressed by the Ukrainian people.
Diplomats in Brussels said that sanctions were agreed unanimously by the foreign ministers of the 28-nation bloc, including Foreign Secretary William Hague.
Measures will be targeted at officials blamed for human rights violations that have occurred during the crackdown on anti-government protests, but the list of those involved has yet to be drawn up. T he scope of sanctions will be adjusted according to developments on the ground.
Speaking ahead of the emergency Brussels meeting, Mr Hague said that Britain condemned the bloodshed in Kiev "in the strongest possible terms" and called for an "immediate and far-reaching change" in attitude of the authorities.
"By permitting such action to take place, the Ukrainian government is putting itself at odds with reasonable opinion all across the world," said Mr Hague.
"It is not right to describe protesters as terrorists. A great many of them are simply seeking a better future for their country."
Earlier the Ukrainian ambassador to London, Volodymyr Khandogiy, was summoned to the Foreign Office for the second time in as many days to be told that the action to crush the protests was "unacceptable".
The EU sanctions were welcomed by Labour leader Ed Miliband, who said on Twitter: "Scenes of violence in Ukraine are utterly appalling. EU sanctions welcome and Government must continue to work with our partners to end bloodshed."
The clashes this week have been the most deadly since protests kicked off three months ago after Mr Yanukovych shelved an association agreement with the European Union in favour of closer ties with Russia. Moscow then announced a multibillion-pound bailout for Ukraine, whose economy is in tatters.
Europe minister David Lidington, who summoned Mr Khandogiy to the Foreign Office, said that he had challenged the ambassador over reports that Ukrainian authorities were behind the latest killings.
"His reaction is that he was not able to give me a definitive view about that," he said.
"I said that these reports were particularly shocking, because if those reports are substantiated then that would suggest that people who were trained in marksmanship were involved, and that does point to people who have been trained officially in some way.
"But I think the key thing there is that, in light of what has happened, there should be a full and independent and thorough investigation of those killings - the people responsible for those shootings need to be held to account, wherever they come from."
Leaving the emergency meeting, Mr Hague said there had been " a strongly united discussion and decision among the EU foreign ministers".
He added: "There is widespread horror in the European Union as well as in the United Kingdom at the scale of the loss of innocent life and the events of the last 48 hours.
"Of course we call on all involved to turn away from violence, but some people are responsible for the violence and so we have decided to introduce targeted measures and targeted sanctions involving visa bans and asset freezes on those individuals who are responsible.
"The scale of the implementation will depend on developments to come and of course we want to see success in government and opposition working together in order to bring about a peaceful situation and a peaceful and democratic settlement of the issues in Ukraine. But that is the decision the council has made today."
Mr Hague said it was also agreed that EU states would make sure they are not supplying to Ukraine "any equipment that can be used for internal repression".
He said the sanctions would send " a strong signal... of how unacceptable this is".
"It should be unacceptable in any city or country in the world, unacceptable in a European city, a European country," said the Foreign Secretary. "And it's a signal of the EU's determination to do something about that."