Ukraine’s acting President Oleksandr Turchynov said the country will launch a "large-scale anti-terrorist operation" to resist Russia's aggression. Turchynov spoke in a live televised address from parliament.
Turchynov said authorities in Kyiv will not allow the "repetition of the Crimean scenario."
Russia annexed the Crimean Peninsula in March after local residents voted to secede from Ukraine and seek to join Russia in a referendum.
In a decree, Turchynov offered not to prosecute any militants who gave up their weapons by 9 a.m. (0600 GMT/UTC) on April 14.
Turchynov's announcement came after pro-Russia militants took over Slovyansk on April 12 and targeted several other municipalities in eastern Ukraine.
Activists also took over police stations and official buildings in other cities in eastern Ukraine, demanding a referendum on autonomy and possibly joining Russia.