By Andrew Osborn and Polina Nikolskaya
MOSCOW (Reuters) -President Vladimir Putin authorised "a special military operation" against Ukraine on Thursday morning to eliminate what he called a serious threat, saying his aim was to demilitarise Russia's southern neighbour.
In an early morning address on state television, Putin said he had been left with no choice but to launch the operation, the scope of which was not immediately clear but appeared to go well beyond helping Russian-backed separatists in eastern Ukraine.
"I have decided to conduct a special military operation," said Putin, seated at a desk in the Kremlin next to a battery of telephones, with the Russian flag behind him.
"Its goal is to protect people who have been subjected to bullying and genocide... for the last eight years. And for this we will strive for the demilitarisation and denazification of Ukraine.
"And to bring to court those who committed numerous bloody crimes against civilians, including against citizens of the Russian Federation."
Ukraine dismisses as invented Russian accusations of genocide against people living in parts of its east seized by Russian-backed separatists in 2014. Kyiv has said Putin was looking for an artificial pretext to attack it.
The Kremlin chief's announcement followed an appeal from the Russian-backed separatists in eastern Ukraine for military help against what they said was growing Ukrainian aggression.
Kyiv has denied any such aggression.
Putin told the Ukrainian military to lay down its weapons and go home.
"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," said Putin who spoke with anger in his voice at times.
A Reuters reporter in Kyiv, the Ukrainian capital, heard explosions that sounded like artillery shelling in the distance minutes after Putin finished speaking.
"Russia cannot feel safe, develop, and exist with
a constant threat emanating from the territory of modern Ukraine," Putin said.
Russia would respond instantly if any external force tried to interfere with its actions, he added.
"Whoever tries to hinder us, and even more so, to create threats to our country, to our people, should know that Russia's response will be immediate. And it will lead you to such consequences that you have never encountered in your history.
"We are ready for any development of events. All the necessary decisions have been made in this regard. I hope that I will be heard."
Putin's comments come after the United States said Russia had stationed nearly 150,000 troops near Ukraine's borders.
Putin repeated past complaints about the failure of NATO and the United States to satisfy Russia's security demands.