A Ukrainian officer has been killed
in a gun battle with pro-Russian armed men in the eastern city of Sloviansk, the
interior minister says.
Both sides suffered a number of casualties, Arsen Avakov said.
Pro-Russian forces took over Sloviansk on Saturday and have targeted at least four other cities, prompting Kiev to launch an "anti-terror operation".
Kiev and Western powers accuse Moscow of inciting the trouble. The Kremlin denies the charge.
US officials said on Saturday there had been a "concerted campaign" by forces with Russian support to undermine the authorities in Kiev.
Secretary of State John Kerry warned of "additional consequences" if Russia failed to make efforts to "de-escalate" and pull its troops back from Ukraine's border.
Ukrainian forces used military helicopters in the
their bid to shift the Sloviansk protesters 
Protesters burned tyres and bolstered their defences
before the government operation began But Russia's Foreign Minister Sergei Lavrov said the Kiev government was "demonstrating its inability to take responsibility for the fate of the country".
Continue reading the main story
The question that everybody is asking right now is what will happen next?
We've heard about diplomatic moves - Russia, the US, the EU and Ukraine are
supposed to meet later this week to try to defuse this crisis. But that looks
like it's under threat.
The ultimate question is what Russia's next move will be. The Kremlin says it has "interests" in eastern Ukraine, where many Russian speakers live. And Russia had already warned Ukraine not to crack down on these militants.
Analysis
The ultimate question is what Russia's next move will be. The Kremlin says it has "interests" in eastern Ukraine, where many Russian speakers live. And Russia had already warned Ukraine not to crack down on these militants.
He had warned earlier that any use of force in eastern
Ukraine could scupper crisis talks due later this week.
Ukrainian officials were due to meet counterparts from Russia, the US and the EU in Geneva on Thursday.
'Shooting to kill'
On Saturday, armed men took over police stations and official buildings in Sloviansk, Kramatorsk and Druzhkovka.
Unconfirmed reports suggested official buildings had also been taken over in two other cities - Mariupol and Yenakievo.
Similar accounts emerged from Sloviansk and Kramatorsk on Saturday of armed men dressed in camouflage arriving in buses and storming the police stations.
Pro-Russian activists have blockaded the security
services' office in Luhansk 
In the regional capital, Donetsk, several public
buildings have been occupied Pro-Russian demonstrators also continued their occupation of the main administrative building in the regional capital Donetsk, which they have held for one week.
Continue reading the main story
Crisis timeline
- Nov 2013: President Viktor Yanukovych abandons an EU deal
- Dec: Pro-EU protests erupt
- 20-21 Feb 2014: Dozens killed in Kiev clashes
- 22 Feb: Mr Yanukovych flees;
- 27-28 Feb: Pro-Russian gunmen seize key buildings in Crimea.
- 16 Mar: Crimea voters choose to secede in disputed referendum: Russia later absorbs region
- Apr: Pro-Russia activists take over government buildings and police stations in eastern Ukraine
A protest leader told the BBC that the activists in
Sloviansk took action to support the Donetsk sit-in.
BBC reporters in Sloviansk said the gunmen were well-organised and quickly established control throughout the town.
Checkpoints had been set up on the main roads into the town.
Mr Avakov labelled the actions a "display of aggression by Russia".
Announcing the operation to clear the activists, he warned people to stay in their homes in Sloviansk.
"The separatists are shooting to kill without warning against the approaching special forces," he said,
He later said Ukrainian forces had been attacked at a checkpoint on the way to Sloviansk, and at least one officer had been killed and five others wounded.
An unknown number of militants were also wounded, he said.
Witnesses at the police station say there is not yet any sign of clashes, and the centre of the town is quiet.
Eastern Ukraine has a large Russian-speaking population and has seen a series of protests since the ousting of Ukraine's pro-Russian President Viktor Yanukovych in February.
Are you in eastern Ukraine? Have you been affected by the unrest? You can email your experiences to haveyoursay@bbc.co.uk using the subject line "Ukraine".
Or send us your experiences using the form
below.
Send your pictures and videos to yourpics@bbc.co.uk or text them to 61124 (UK) or +44 7624 800 100 (International). If you have a large file you can upload here.
