The European Union has imposed sanctions on Ukraine, after security forces were blamed for sniper fire which killed dozens of anti-government protesters on the streets of the country's capital, Kiev.
As bloody clashes continued for a second day, Prime Minister David Cameron embarked on a round of telephone diplomacy, speaking to world leaders including Russian President Vladimir Putin, who has been the strongest international backer of the regime of Ukrainian President Viktor Yanukovych.
Unconfirmed reports from local medics suggested that at least 70 died and as many as 500 more were injured as marksmen picked off demonstrators, after firebombs were thrown at riot police.
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 Mr 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. No American sanctions were announced, but a spokesman said President Barack Obama was considering the options open to him.
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.
The Foreign Office has updated its travel advice for Ukraine to advise against all but essential travel to Kiev. UK nationals currently visiting or living in Kiev are advised to stay inside.
Downing Street said Mr Cameron was seeking to build support for a roadmap for a peaceful resolution proposed by European foreign ministers visiting Kiev earlier today.
Following his phone conversation with the Russian leader, Number 10 said: "The Prime Minister agreed with President Putin that they should both encourage all sides in Ukraine to get behind this emerging plan as a way to end the violence and open the way to a lasting peaceful solution."
But Russia's foreign minister, Sergey Lavrov, accused the West of applying "double standards" in the conflict, telling a press conference in Baghdad: "The opposition cannot or does not wish to distance itself from extremist groups.
"Our Western partners and everyone in Europe and the US put all the blame on the government of Ukraine and they do not condemn as they should the actions of the extremists. We are very troubled by all that, because the double standards are obvious here."
Mr Cameron - who described the situation in Ukraine as "deeply concerning" - also spoke to German Chancellor Angela Merkel and Donald Tusk, the Prime Minister of Ukraine's western neighbour Poland. He will discuss the crisis with Dutch PM Mark Rutte when he visits the UK tomorrow.
Downing Street said Mr Cameron and Mrs Merkel were both "extremely concerned by the spiralling violence" and agreed that President Yanukovych had "a particular responsibility to help de-escalate the situation by pulling back government forces".
The PM and Mr Tusk agreed that the 28-nation bloc should "continue to look at the ways it can promote a peaceful and democratic settlement in Ukraine, recognising that continued violence will make it harder to reassure all Ukrainians that their legitimate aspirations will be realised", said Number 10. A spokesman said that all three leaders gave their backing to the EU foreign ministers' roadmap.
Following the emergency meeting in Brussels, the EU's High Representative for Foreign Affairs, Baroness Ashton, said that targeted visa restrictions and asset freezes would be implemented "as a matter of urgency". Details of the individuals to be targeted were not immediately available.
EU nations also agreed to suspend export licences for equipment which could be used for internal repression, said Lady Ashton.
Mr Hague said the sanctions would send a "strong signal" of the unacceptability of the actions of those responsible for the bloodshed on the streets of Kiev.
"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."
Mr Hague 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."
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".
Europe Minister David Lidington said Mr Khandogiy was unable to give "a definitive view" over claims that the Ukrainian authorities were behind the latest killings.
"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," said Mr Lidington.
"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."
Shadow foreign secretary Douglas Alexander welcomed the EU sanctions, adding: "The UK Government must now urgently implement the agreed restrictions, in particular asset freezes and visa bans which could take immediate effect.
"The Ukrainian authorities bear a heavy responsibility to de-escalate the crisis and must now open a fresh and serious dialogue with the opposition."