Russian President Vladimir Putin has authorised a special military operation in Ukraine's Donbass region 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 early on Thursday.
"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.
But US President Joe Biden called Russia's decision to commence a military operation in eastern Ukraine an "unprovoked and unjustified attack".
"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering. Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way.
"The world will hold Russia accountable," Biden said.
Biden said the US will coordinate with its Nato allies to ensure a strong, united response to deter any aggression against the alliance.
Early on Thrusday explosions also rocked the breakaway eastern Ukrainian city of Donetsk early on Thursday and civilian aircraft were warned away as the United States said a major attack by Russia on its neighbour was imminent.
Ukraine restricted civilian flights in its airspace due to "potential hazard", hours after a conflict zone monitor warned airlines should stop overflights over the risk of an unintended shoot-down or cyber attack.
Convoys of military equipment including nine tanks were seen moving towards Donetsk earlier on Wednesday from the direction of the Russian border, a Reuters witness reported.
Shelling has intensified since Russian President Vladimir Putin this week recognised two separatist regions as independent and ordered the deployment of what he called peacekeepers, a move the West calls the start of an invasion.
The UN Security Council met to discuss the stand-off on Wednesday night, in the 15-member body's second late night meeting on the crisis this week.
Satellite imagery taken on Wednesday showed new deployments in western Russia, many of them within 15km of the border with Ukraine and less than 80km from the Ukrainian city of Kharkiv, a US company Maxar said.
A 30-day state of emergency in Ukraine restricting the freedom of movement of conscripted reservists, curbing the media and imposing personal document checks, according to a draft text, begins on Thursday.
The Ukrainian government has also announced compulsory military service for all men of fighting age.
While both sides have said they are still open to diplomacy to resolve the crisis, Ukrainian President Volodymyr Zelenskiy said earlier Moscow had approved an offensive and not replied to an invitation for talks.
"Today I initiated a telephone conversation with the President of the Russian Federation. The result was silence," he said.
Ukrainian government websites, which have experienced outages in recent weeks blamed by Kyiv on cyber attacks, were again offline on Wednesday. Ukraine's parliament, cabinet and foreign ministry websites were affected.
with PA
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