-Russian President Vladimir Putin authorised a special military operation in Ukraine's Donbass region in the early hours of Thursday morning and told the Ukrainian military to lay down its weapons and go home.
In a special televised address on Russian state TV, Putin said Russia has been left with no choice but to defend itself against what he said were threats emanating from modern Ukraine.
Putin also made an appeal to the Ukrainian military.
"I urge you to immediately lay down your weapons and go home. All servicemen of the Ukrainian army who fulfil this demand will be able to freely leave the combat zone and return to their families."
The scope of the Russian military operation in Ukraine was not immediately clear. A Reuters reporter in Kyiv, the Ukrainian capital, heard explosions which sounded like artillery shelling in the distance soon after Putin had finished speaking.
Putin said Russia would respond instantly if any external force tried to interfere and that Moscow would try to de-militarise and 'de-Nazify' Ukraine.
"Russia cannot feel safe, develop, and exist with
a constant threat emanating from the territory of modern Ukraine," said Putin.
His comments come after the United States said Russia had stationed nearly 150,000 troops near Ukraine's borders and after Russian-backed separatists appealed to him for military help against what they said was growing Ukrainian aggression.
Kyiv has denied any such aggression.
"All responsibility for bloodshed will be on the conscience of the ruling regime in Ukraine," Putin told state TV.
"No one should have any doubt that a direct attack on our country will lead to defeat and terrible consequences for any potential aggressor."