Russian President Vladimir Putin on Thursday announced a military operation in Ukraine with explosions audible across the country and warned the foreign minister that a "complete invasion" was underway.

Weeks of intense diplomacy and the imposition of Western sanctions on Russia have failed to deter Putin, who has gathered 150,000 to 200,000 troops on Ukraine's borders.

"I have made the decision of a military operation," Putin said in a surprise television announcement that prompted the immediate condemnation of US President Joe Biden and put global financial markets in chaos.

Shortly after the announcement, according to AFP reporters, explosions were heard in the capital of Ukraine, Kyiv and several other cities.

Raid sirens have been reported in the centre of Kyiv.

Putin called on Ukrainian troops to lay down their arms and justified the operation by saying the government was leading a "genocide" in the east of the country.

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The Kremlin has previously said that rebel leaders in eastern Ukraine have asked Moscow for military aid against Kyiv.

The extent of Thursday's attacks was not immediately clear, but Ukrainian Foreign Minister Dmytro Kuleba said the worst-case scenario was at stake.

"Putin has just launched a full-scale invasion of Ukraine. Peaceful Ukrainian cities are under strikes," Kuleba tweeted.

"This is a war of aggression. Ukraine will defend itself and will win. The world can and must stop Putin. The time to act is now."

Biden immediately warned of "consequences" for Russia, and there would be "devastating loss of life and human suffering."

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The NATO chief condemned Russia's "multiple and senseless attack" on Ukraine.

Putin's move comes after Ukrainian President Volodymyr Zelensky on Wednesday night emotionally appealed to Russia not to support the "major war in Europe."

Zelensky said that the people of Russia were lying about Ukraine.

Zelensky said he had tried to call Putin, but "there was no answer, just silence," adding that Moscow now had nearly 200,000 troops at the Ukrainian border.

Earlier on Wednesday, separatist leaders in Donetsk and Lugansk sent separate letters to Putin, asking him to "help them curb aggression in Ukraine," Putin's spokesman Dmitry Peskov said.

Both letters were published in the Russian state media and both were dated February 22.

Their challenge came after Putin recognized their independence and signed friendship agreements with her, which included defence agreements.

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For weeks, Putin has spoken out against a flood of international criticism of the crisis, with some Western leaders saying he is no longer justified.

His announcement of a military operation precedes the final summit with European Union leaders in Brussels, scheduled for Thursday.

The bloc of 27 countries also imposed sanctions on Russian Defence Minister Sergei Shoigu and senior officials, including commanders of the Russian army, navy and air force, another part of the wave of punishments in the West after Putin crossed Ukraine's borders to be rewritten.

Last Wednesday, due to the crisis, the UN Security Council met for a second state of emergency in three days, when the personal request of UN Secretary-General António Guterres to Putin was not heard.

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"President Putin, stop your troops from attacking Ukraine, give peace a chance, too many people have already died," Guterres said.

US Ambassador to the UN Linda Thomas-Greenfield warned that a complete invasion of Russia could displace five million people, which could trigger a new refugee crisis in Europe.

Before Putin's announcement, Ukraine called on almost three million citizens living in Russia to leave.

"We are united in believing that the future of European security is being decided right now, here in our home, in Ukraine," President Zelensky said during a joint media appearance with the visiting leaders of Poland and Lithuania.

The Western capital claims that Russia has assembled 150,000 troops in combat formations on the Ukrainian borders with Russia, Belarus and Crimea occupied by Russia and on Black Sea warships.

Ukraine has almost 200,000 troops and can recruit up to 250,000 midfielders.

Moscow's total strength is much larger - almost a million active personnel - and has been modernized and re-armed in recent years.