Ukraine has mobilised its army to fight "terrorists" after gun battles broke out in the city of Slavyansk as separatists seized more government buildings.
The country's acting president, Oleksandr Turchynov, said he would "not allow Russia to repeat the Crimean scenario in the eastern regions".
One officer was killed and another five injured as security forces attempted to retake the security buildings, according to Interior Minister Arsen Avakov.
Mr Avakov said there had been an "unidentifiable number" of casualties among the protesters in the eastern city.
Russia's RIA news agency confirmed at least one activist had been killed in the raid.
Pro-Russian separatists are guarding barricades outside the occupied buildings and have set up checkpoints with "human shields" on roads leading into Slavyansk.
So far there has been no sign of the threatened army offensive - but people in the city have been warned to stay inside.
The government has reportedly issued a deadline of 6am GMT for the separatists to lay down their weapons.
Acting president Oleksandr Turchynov accused Russia of stoking the unrest to justify expanding its military control beyond Crimea, a claim rejected by the Kremlin.
"Each of those who support the aggressors and occupiers is conducting armed resistance against our state," added Mr Turchynov in a televised address.
At Russia's request, the UN Security Council is holding an emergency session in New York later to discuss the crisis. The country says any armed crackdown against the separatists would be "criminal".
The US meanwhile has threatened to bolster existing sanctions against Moscow if the trouble continues. It too blames Russia for stoking the latest violence.
"I think we've seen that the sanctions can bite," said Samantha Power, America's ambassador to the UN.
"If actions like the kind we've seen over the last few days continue, you're going to see a ramping up of those sanctions."
Sky’s Katie Stallard, who is in Slavyansk, said the centre remains relatively calm and reports of fatalities were thought to be linked to clashes well into the outskirts.
She reported that civilians, some of them elderly, are stood at one checkpoint to act as "human shields" to protect protesters from Ukrainian security forces.
Elsewhere, protesters are also said to 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 were erected around the office and the Ukrainian flag removed.
If confirmed, that would bring the total number of cities where administrative buildings are under the control of separatists to five.
Masked men, wearing unmarked camouflage uniforms and carrying special-issue assault rifles, 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 residents fear they will be suppressed under the new Western-friendly interim government in Kiev.
Moscow has also warned any Ukrainian military aggression against ethnic Russians will derail top-level talks on the crisis scheduled to take place in Geneva 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.
The US has also called on Moscow to withdraw its troops from Ukraine's eastern border, where Nato claims up 40,000 soldiers are now stationed.
The EU and US have imposed a number of sanctions on Russian and Ukrainian individuals, but are yet to directly target Russia's economy.