The separatist Donetsk People's Republic has declared civil war in Ukraine and called for a general mobilisation of force in the country's east, in retaliation for a government "anti-terror operation" in the city of Slaviansk.
"A combined arms operation has been launched in Slaviansk. This means only one thing: a civil war," the leadership of the Donetsk People's Republic told Interfax news agency.
According to the Ukrainian Interior Ministry, five pro-Russian militants were killed in the Slaviansk operation.
"During the clashes five terrorists were killed," a spokesman said, adding that a soldier was also wounded.
Russian president Vladimir Putin, speaking on Russian TV, condemned the military operation as a criminal act, saying: "If the regime in Kiev has begun using the army against the population inside the country, then this is undoubtedly a very serious crime."
"If these people have advanced to the so called "acute phase" (of confrontation with protesters), this is not an acute phase, it is just a punitive operation and it will of course incur consequences for the people making these decisions, including (an effect) on our interstate relations," he continued.
Mayor threatens to shoot civilians
The mayor of Slaviansk, Vyacheslav Ponomaryov, has threatened to shoot civilians if they work with Ukrainian law enforcement agencies or if they are caught with pro-government flyers.
The flyers distributed in Slaviansk have ordered residents to "stay indoors and not let children out onto the streets without parental supervision, to not succumb to provocations, and not follow orders of the self-proclaimed illegal government," according to the Kiev Post.
Elsewhere, Ukrainian government forces have regained control of the Mariupol city hall from pro-Russian separatists in the eastern port city.
Russia annexed the Crimea region last month for the reason that Russian speakers were threatened by unrest in Ukraine. Russian leaders have warned they will take similar action in eastern Ukraine if Russian speakers become endangered in any way.
The Donetsk People's Republic is a self-proclaimed state in eastern Ukraine by pro-Russian activists who have taken control of key administrative buildings in Donetsk and other prominent cities in the region.