AFP correspondents also heard blasts in the Black Sea port city of Odessa, with police and ambulance sirens heard across the Ukrainian capital city.

Explosions also rang out in Kharkiv, a large city 35 kilometres south of the Russian border.

Four loud blasts rang out in Kramatorsk, a frontline city that serves as the Ukrainian government's effective capital for the eastern war zone, and more were heard in the eastern port city of Mariupol, AFP reporters said.

Ukraine closed its airspace to civilian planes, and Foreign Minister Dmytro Kuleba tweeted that his country was facing a "full-scale invasion"'.

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Russian President Vladimir Putin announced on Thursday a military operation in Ukraine to defend separatists in the east of the country, and "demilitarise and de-nazify" its pro-Western neighbour.

"I have made the decision of a military operation," he said in a surprise statement on television shortly before 6 am (0300 GMT).

He went on to denounce what he called a "genocide" orchestrated by Ukraine in the country's east, as well as NATO's aggressive policy towards Russia.

"For this, we will strive to achieve demilitarisation and de-Nazification of Ukraine," said Putin, promising to bring "to court those who have committed many crimes, responsible for the bloodshed of civilians, including Russian citizens".

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The Russian leader addressed the Ukrainian military, calling on soldiers to "lay down your arms", before issuing an assurance that they could "leave the battlefield without hindrance".

He said that he did not want an "occupation" of Ukraine, but its "demilitarisation". 

Putin then addressed those "who would try to interfere with us... they must know that the response of Russia will be immediate and will lead to consequences that you have never known before". 

"I am sure that the soldiers and officers of Russia will fulfill their duty with courage," he said, adding "the security of the country is guaranteed." 

The Russian leader did not specify the scope of the military operation, or whether it would be limited to eastern Ukraine. 

AFP/eNCA