Pro-Russian protesters have stormed several local government buildings in the eastern Ukrainian city of Luhansk.
Reports say the group opened fire with automatic weapons at the city's police headquarters.
Ukraine's interim president, Olexander Turchynov, has criticised police for their inaction, the BBC reports.
Moscow has said it has no intention of invading eastern Ukraine, where pro-Russia activists have seized government buildings in more than a dozen towns and cities.
However, the government in Kiev has all but lost control of its police forces in parts of the region.
Meanwhile, Russia has condemned new sanctions imposed by the United States and the European Union for what they say is Moscow's destabilisation of Ukraine.
The Kremlin has described the measures as reminiscent of the Iron Curtain era.
Secretary of State John Kerry told the Kremlin to "leave Ukraine in peace" and warned: "Nato territory is inviolable we will defend every single inch of it."