The BBC's David Stern reports: Moscow denies the trouble has been ''orchestrated'' from the Kremlin
Ukraine's president says a full-scale operation involving the army will be launched in the east after pro-Russian militants seized government buildings.
Acting President Oleksandr Turchynov said he would not allow a repetition of what happened in Crimea which was annexed by Russia last month.
His live televised address from parliament came after pro-Russian forces targeted half a dozen cities.
Earlier, Nato's secretary general voiced concern at events in the region.
And the US ambassador to the UN said the attacks this weekend bore the "tell-tale signs of Moscow's involvement." But the Kremlin denies involvement in events in eastern Ukraine.
Early on Sunday Ukrainian authorities said they launched an "anti-terror operation" after armed men took over the city of Sloviansk.
A Ukrainian officer was killed in a gun battle in the city, and there are reports the Ukrainian operation has been halted.
But both sides suffered a number of casualties, interim Ukrainian Interior Minister Arsen Avakov said.
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.
'Sowing discord'
"We will not allow Russia to repeat the Crimean scenario in the eastern regions of Ukraine," said President Turchynov.
"The aggressor... is continuing to sow disorder in the east of the country."
But Mr Turchynov offered not to prosecute militants who gave up their weapons by early Monday.
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 beganNato chief Anders Fogh Rasmussen's statement on Sunday drew parallels with some aspects of last month's seizure of Crimea.
He said the "reappearance of men with specialised Russian weapons and identical uniforms without insignia, as previously worn by Russian troops during Russia's illegal and illegitimate seizure of Crimea, is a grave development".
A Nato source told the BBC the organisation believed that "Russian forces have been involved in the seizure of some of the buildings".
And the US ambassador to the UN said the attacks on police and other buildings in eastern Ukraine had "telltale signs of Moscow's involvement".
"It's professional, co-ordinated. Nothing grass-roots about it," ambassador Samantha Power told ABC News.
"The forces are doing in each of the six or seven cities they have been active in exactly the same thing."
Turning violent
On Saturday, armed men took over police stations and official buildings in Sloviansk, Kramatorsk and Druzhkivka.
Similar accounts emerged of armed men dressed in camouflage arriving in buses in Sloviansk and Kramatorsk and storming the police stations.
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.
Pro-Russian activists have blockaded the security services' office in Luhansk
In the regional capital, Donetsk, several public buildings have been occupied
In the port city of Mariupol, the Ukrainian flag was removed at an official buildingIn other developments:
- Rival rallies turned violent in the eastern city of Kharkiv - Ukraine's second biggest - with reports of 10 people injured
- Pro-Russian activists wielding clubs surround Kharkiv's city council, with mayor Henadiy Kernes reportedly inside
- Unconfirmed reports suggested official buildings had also been taken over in two other cities - Mariupol and Yenakievo.
- Pro-Russian demonstrators continued occupying the main administrative building in the regional capital Donetsk, which they have held for one week
- A protest leader told the BBC that the activists in Sloviansk took action to support the Donetsk sit-in.
There was heavy gunfire as armed men took the police station in Kramatorsk
Interior Minister Avakov labelled the weekend 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 said there was no sign yet of any clashes, and the centre of the town was quiet.
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".
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.
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