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.

The scope of the Russian military operation was not immediately clear. A Reuters reporter in Kyiv, the Ukrainian capital, heard explosions which sounded like artillery shelling in the distance.

Putin said Russia would respond instantly if any external force tried to interfere with its actions and that Moscow would try to de-militarise and ‘de-Nazify’ Ukraine.

Putin’s 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.