Pro-Russia protesters have seized government buildings in the eastern part of Ukraine.
The United States and the European Union intend to work out new sanctions against Russia.
On Sunday, voters in the Crimea region voted with overwhelming majority to separate from Ukraine to join Russia, as the region has an ethnic Russian majority and became part of Ukraine only 60 years ago.
Crimean lawmakers have voted Thursday in favor of the Ukrainian region becoming part of Russia, and its pro-Moscow government has scheduled a referendum on March 16.
Ukraine mobilizes its forces after Russian president claimed he had the right to invade in the neighbouring state.
Ukraine new government asked NATO for help as Russian troops entered the region of Crimea.