Deadly gun battles have broken out in the eastern Ukrainian city of Slavyansk as separatists seized government buildings in at least one other city in the region.
Ukraine’s Interior Minister Arsen Avakov said one officer was killed and another five injured as security forces attempted to retake occupied security buildings.
In a statement on his Facebook page Mr Avakov said there was also an “unidentifiable number” of casualties among the protesters, who are being supported by around 1,000 civilians.
He earlier announced the government had launched an “anti-terrorism operation,” urging residents in the city to stay indoors.
Around 1,000 civilians have reportedly come out in support of the activists
Russia’s RIA news agency confirmed that at least one activist had been killed in the raid.
Separatists are currently guarding barricades outside the occupied buildings and have set up checkpoints on roads leading into the city.
Sky’s Katie Stallard reported that civilians, some of them elderly, have been stood at one checkpoint on the outskirts of Slavyansk since late last night to act as “human shields” to protect pro-Russian protesters from Ukrainian security forces.
Elsewhere local media is reporting that protesters have seized the mayor’s office in the eastern city of Mariupol.
One report said activists entered the building unhindered after around 1,000 people took part in an independence rally.
It said barricades have been erected around the office and the Ukrainian flag has been removed.
If confirmed that would bring the total number of cities where administrative buildings are under the control of separatists to five.
Masked men, carrying automatic weapons, seized the local police headquarters in Slavyansk on Saturday.
They later took control of the security service building in the city, which lies some 90 miles from the Russian border.
Also on Saturday gunmen seized a police station in the eastern city of Kramatorsk following a shoot-out.
Other groups began their occupation of security buildings in Donetsk and Luhansk last weekend.
Protesters in the region, which has a large ethnic Russian population, hope to follow in the footsteps of the Crimean Peninsula and force a referendum on joining the Russian Federation.
The area was a strong bastion of support for ousted President and Kremlin ally Viktor Yanukovych and many fear they will be suppressed under the new Western-friendly interim government.
Military helicopters are currently surveying the area
Ukraine has accused Russia of orchestrating the unrest in order to justify expanding its military control beyond Crimea.
Ukraine’s interior minister said he considered the attacks an “act of aggression by Russia”.
However Russian President Vladimir Putin strenuously denies stoking tensions.
Moscow has also warned that any Ukrainian military aggression against ethnic Russians will derail top-level talks on the crisis scheduled to take place next week.
Western leaders have expressed concern that the latest unrest strongly echoes the events which led to Russia’s annexation of Crimea last month following an overwhelmingly-backed referendum.