US President Joe Biden has called Russia’s decision to commence a military operation in eastern Ukraine an “unprovoked and unjustified attack”.

“The prayers of the entire world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack by Russian military forces,” he said shortly after confirmation of widespread attacks across Ukraine on Thursday afternoon (Australian time).

“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.”

In an early morning address from Moscow, Russian President Vladimir Putin said he had authorised “a special military operation” in Ukraine’s Donbass region and told the Ukrainian military to lay down its weapons and go home.

Mr 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 in Ukraine was not immediately clear.

A Reuters reporter in Kyiv, the Ukrainian capital, heard explosions that sounded like artillery shelling in the distance soon after Mr Putin had finished speaking. There were also reports of more blasts in Ukraine’s capital Kyiv and its second-largest city of Kharkiv.

Interfax Ukraine reported rocket attacks on military facilities across the country, Ukraine and that Russian troops had landed in the southern port cities of Odessa and Mariupol.

It also reported staff and passengers evacuating Kyiv’s Boryspil airport.

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Ukrainian President Volodymyr Zelensky urged people to remain calm in a brief video address on Thursday morning, as he also imposed martial law.

“This morning President Putin announced a special military operation in Donbas. Russia conducted strikes on our military infrastructure and our border guards. There were blasts heard in many cities of Ukraine,” he said.

“A minute ago I had a conversation with President Biden. The US have already started uniting international support.

“Today each of you should keep calm. Stay at home if you can. We are working. The army is working. The whole sector of defence and security is working.

“We are strong. We are ready for everything. We will win over everybody because we are Ukraine.”

Mr 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,” he said.

He said Russia had “no plans” to occupy Ukraine, and that the Ukrainian people would be able “to choose freely” who ran the country.

The Russian leader also made an appeal to the Ukrainian military in his address, shown on Russian state TV.

“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,” he said.

“All responsibility for bloodshed will be on the conscience of the ruling regime in Ukraine.”

Mr Biden said that he planned to speak to other G7 members and would address Americans later on Thursday. He said he intended to announce “further consequences” that the US and its allies planned to impose on Russia over “this needless act of aggression against Ukraine and global peace and security.”

“The world will hold Russia accountable,” Mr Biden said.

“I will be monitoring the situation from the White House this evening and will continue to get regular updates from my national security team.

Mr Biden said the US would also coordinate with its Nato allies to ensure a strong, united response to deter any aggression against the alliance.

“Tonight, Jill and I are praying for the brave and proud people of Ukraine,” he said.

Ukraine’s foreign minister Dmytro Kuleba accused Mr Putin of launching a full-scale invasion, and said his country would defend itself.

“The world can, and must, stop Putin,” he said in a tweet on Thursday.

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Confirmation of the Russian attacks came as the UN Security Council held an emergency meeting late on Wednesday in New York (local time). Ukraine’s Ambassador to the United Nations Volodymyr Yelchenko directly questioned Russia’s representative in his speech.

“About 48 minutes ago, your president declared war on Ukraine,” Mr Yelchenko said.

“You declared the war. It is the responsibility of this body to stop the war.”

Russia is the current chair of the council, and one of the permanent members that has a power of veto.

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Early on Thursday explosions also rocked the breakaway eastern Ukrainian city of Donetsk and civilian aircraft were warned away as the US 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.

A 30-day state of emergency in Ukraine began on Thursday. The Ukrainian government has also announced compulsory military service for all men of fighting age.

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 AAP