Vladimir Putin has launched an invasion on Ukraine.
Explosions have been heard in the Ukrainian capital of Kiev early this morning after the Russian president's announcement that a military operation had begun.
During a televised address early on Thursday morning, Mr Putin said the move was a response to threats from Ukraine.
READ MORE: LIVE Putin launches Russian invasion of Ukraine and explosions rock cities
The Ukrainian Minister of Foreign Affairs, Dmytro Kuleba, said Russia has launched a "full-scale invasion of Ukraine."
As a result, Ukraine's president has declared martial law and urged its citizens not to panic.
Boris Johnson is leading a crisis meeting of senior ministers and officials to consider how to respond to Vladimir Putin's military action against Ukraine.
The Prime Minister said the Russian president has "chosen a path of bloodshed and destruction."
Mr Johnson said: "I am appalled by the horrific events in Ukraine and I have spoken to President Zelensky to discuss next steps.
"President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine.
"The UK and our allies will respond decisively."
What is the Ukraine crisis? Russia-Ukraine relations
Vladimir Putin has denied planning to attack Ukraine for months but this week he announced a 'special military operation' in the country's Donbas region.
The operation comes after the president tore up a peace deal and ordered troops into two rebel-held eastern regions.
The conflict began in 2014 when the breakaway regions were seized by Russian-backed rebels.
Mr Putin launched that attack after mass street protests in Ukraine that ousted pro-Russian President Viktor Yanukovych.
Since then more than 14,000 people have died in the conflict with Ukraine.
A ceasefire was held and there was an international Minsk peace accord however the Russian leader sent troops into the rebel-held areas when conflict continued.
What does Putin want?
Mr Putin said the military operation's objective was to defend the people in the breakaway areas.
He accused the US and its allies of ignoring Russia's demands to block Ukraine from ever joining Nato and offer Moscow security guarantees.
Mr Putin said Russia does not intend to occupy Ukraine but will "demilitarise" it. Soon after his address, explosions were heard in the cities of Kiev, Kharkiv and Odesa. Russia said it was attacking military targets.
He urged Ukrainian servicemen to "immediately put down arms and go home."
Ukraine's border guard agency said the Russian military has attacked from neighbouring Belarus, unleashing a barrage of artillery. The agency said Ukrainian border guards fired back, adding that there was no immediate report of casualties. Russian troops have been in Belarus for military drills.
Martial law meaning
Ukraine's president has declared martial law and urged its citizens, not to panic.
Martial law is a temporary rule by military authorities of a designated area in time of emergency when the civil authorities are deemed unable to function.
Ukrainian ambassador Sergiy Kyslytsya has asked for another emergency meeting of the Security Council, calling on the UN body "to stop the war because it's too late to talk about de-escalation".
Where in Ukraine is Russia invading?
Ukraine's border guard agency said the Russian military attacked the country from neighbouring Belarus.
The agency said the Russian troops unleashed an artillery barrage as part of an attack backed by Belarus and Ukrainian border guards were firing back, adding that there was no immediate report of casualties.
Russia's military said it had targeted Ukrainian air bases and other military assets and had not targeted populated areas.
Explosions were heard in the Ukrainian capital of Kyiv as well as Odesa and Kharkiv following Vladimir Putin's announcement that the military operation had begun.
Russia first launched strikes on Ukraine's military infrastructure and border guard units, according to Mr Zelensky. Then Ukrainian forces said Russian military vehicles had crossed the border at Kharkiv in the north, Luhansk in the east, Russian-annexed Crimea in the south and from Belarus too.
Ukraine's army said Kyiv's Boryspil international airport was among a number of airfields that had been bombed, along with military headquarters and warehouses in the big cities of Kyiv, Dnipro, Kharkiv and Mariupol.
Mr Zelensky said Russia had positioned almost 200,000 troops and thousands of combat vehicles on Ukraine's borders.
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