Russia has ordered new military exercises on its border with Ukraine in response to an operation by Kiev against pro-Russian separatists.
Russian Defence Minister Sergei Shoigu said Moscow had been "forced to react" after Ukrainian commandos moved on the separatist stronghold of Sloviansk.
President Vladimir Putin had earlier warned Ukraine of "consequences".
Pro-Russian separatists are occupying key buildings in a dozen eastern towns, defying the central government.
Mr Shoigu was quoted by the Interfax news agency as saying: "If this military machine is not stopped, it will lead to greater numbers of dead and wounded.
"We are forced to react to such a development of the situation."
Russian Defence Minister Sergei Shoigu said Moscow had been "forced to react" after Ukrainian commandos moved on the separatist stronghold of Sloviansk.
President Vladimir Putin had earlier warned Ukraine of "consequences".
Pro-Russian separatists are occupying key buildings in a dozen eastern towns, defying the central government.
Mr Shoigu was quoted by the Interfax news agency as saying: "If this military machine is not stopped, it will lead to greater numbers of dead and wounded.
"We are forced to react to such a development of the situation."
