Sanctions against Russia are to be expanded with more people added to the list subject to asset freezes and visa bans, Baroness Ashton has said.
The European Union's foreign affairs chief said the action had been agreed in light of the latest events, as she also urged Russia to pull back its troops from the Ukrainian border.
Speaking after talks in Luxembourg with EU foreign ministers, Baroness Ashton told reporters: "We condemn unreservedly the actions by armed individuals in the cities of eastern Ukraine.
"These attempts at destabilising Ukraine must stop. We strongly support the unity, sovereignty and territorial integrity of Ukraine.
"We call upon Russia to do so as well and pull back its troops from the Ukrainian border.
"Any further actions aimed at destabilising Ukraine have to stop.
"In light of the latest events we decided to expand the list of those subject to asset freezes and visa bans.
"Utmost restraint is crucial. We commend the Ukrainian authorities for pursuing the law and order operations in a measured manner and we encourage the government in Kiev to contribute further to reducing tensions.
"Free and fair presidential elections on 25 May are the best way to express the will of the citizens as is the process of constitutional reform."
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The EU - which has already imposed a series of asset freezes and travel bans - earlier warned that Russia faces more stringent economic sanctions if it further destabilises the situation.
Britain has also accused Russia of a "deliberate destabilisation" of Ukraine as pro-Moscow separatists continued to seize yet another police building in the east of the country.
Arriving for talks in Luxembourg with EU foreign ministers, foreign secretary William Hague called for a "clear and united international response" to the latest violations.
He said that EU officials needed to press ahead with preparations for a new wave of hard-hitting economic sanctions against Russia if it continued its efforts to undermine its neighbour.
"There do have to be consequences to a further and further escalation by Russia of this crisis," he told reporters.
Moscow has denied orchestrating the latest wave of seizures of police stations and other public buildings in the largely pro-Russian eastern Ukraine, but Mr Hague said their protestations lacked a "shred of credibility".
"There can't be any real doubt that this is something that has been planned and brought about by Russia," he said.
"It has all the appearances of a further gross, deliberate and premeditated violation of the independence and sovereignty of Ukraine.
"It is also clearly a very dangerous thing to do and therefore there has to be a clear and united international response to that.
"I don't think denials of Russian involvement have a shred of credibility."
David Cameron interrupted his Easter holiday in Lanzarote to speak separately with German Chancellor Angela Merkel and French President Francois Hollande about the worsening situation.
Mr Hague said the forces involved in the seizures were behaving in exactly the same way as Russian troops in Crimea before Moscow's annexation of the peninsula.
Speaking at a Whitehall news conference, deputy prime minister Nick Clegg said: "We have been very clear that we want to pursue a consistent approach, a ratchet-type approach such that measures will be followed with further measures if de-escalation does not take place."
However Mr Hague acknowledged that would require the agreement of all 28 member states - with some countries concerned that Moscow could retaliate by restricting gas supplies.