Russian forces have launched a military assault on neighboring Ukraine, crossing its borders and bombing military targets near big cities, the BBC reports.

Russian President Vladimir Putin in a pre-dawn TV statement on Thursday morning, said he did not plan to occupy Ukraine and demanded that its military lay down their blood.

Moments later, attacks were reported on Ukrainian military targets and Ukraine said, 'Putin has launched a full-scale invasion of Ukraine'.

BBC reports that Russia's military breached the border in several places, in the north, south, and east, including from Belarus, a long-time Russian ally. There are reports of fighting in some parts of eastern Ukraine with about 10 civilians believed to have been killed, including six in an airstrike in Brovary near the capital Kyiv.

A Ukrainian presidential adviser said that more than 40 soldiers had died and dozens more were wounded. Ukraine said it had killed 50 Russian troops and shot down six Russian aircraft, but this has not been verified, according to the BBC.

Ukrainian President Volodymyr Zelensky announced martial law across all of Ukraine, severed all diplomatic relations with Russia, and said weapons would be distributed to anyone who wanted them.

"No panic. We're strong. We're ready for anything. We'll defeat everyone because we are Ukraine," he said in a video statement. Ahead of Russia's attack, he had made a last-ditch attempt to avert a conflict, warning that Russia could start "a major war in Europe" and urging Russian citizens to oppose it.

The Russian leader launched the "special military operation" by repeating a number of unfounded claims he has made this week, including alleging that Ukraine's democratically elected government had been responsible for eight years of genocide.

He said the goal was demilitarisation and "denazification" of Ukraine. 

Mr. Putin also warned that any outside power intervening on Ukraine's behalf would face an "instant" response.

Russia's action has received international condemnation.

In a televised address, UK Prime Minister Boris Johnson said that the "hideous and barbaric venture by Vladimir Putin must end in failure".

Addressing Russians, he said: "I cannot believe this is being done in your name, or that you really want the pariah status it will bring to the Putin regime." He told Ukrainians that the UK was "on your side".

US President Joe Biden said the world would hold Russia accountable. He is expected to address Americans on Thursday about the consequences Russia will face.

France's President Emmanuel Macron said the attack would have "deep, lasting consequences for our lives".

EU foreign policy chief Josep Borrell said "these are among the darkest hours for Europe since World War Two".