The blunt warning from Russia's paramount leader, whose country has more nuclear warheads than even the United States, marks the biggest escalation of the war in Ukraine since Moscow's Feb. 24 invasion.
Putin said he had signed a decree on a partial mobilisation.
The mobilisation, which affects anyone who has served as a professional soldier in Russia rather than a conscript, begins immediately.
Putin said his aim was to "liberate" east Ukraine's Donbas region, and that most people living in regions under Russian control did not want to be ruled by Kyiv anymore.
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