Putin's tanks roll into Ukraine over Belarus border as he launches full-scale invasion

Border guard units said they were fighting for control of the eastern city of Sumy against Russian forces while fierce battles were also raging in Donetsk and Luhansk against pro-Russian rebel forces who launched an attack from rebel-held enclaves.

Estimates of dead and injured were almost non-existent as of early afternoon. One Ukrainian official said 'hundreds' had been killed in early fighting, while another put the death toll at 40 Ukrainians and 50 Russians.

The port cities of Mariupol and Odessa, where Ukraine's main naval bases are located, were also attacked - though Odessa appeared to remain under Ukrainian control as of Thursday afternoon. Russian tankers blockaded the Kerch Strait, leading from the Back Sea to the Sea of Azov, cutting off Mariupol.

Volodymyr Zelensky, in an address to the nation on Thursday morning, said the history of Ukraine has now changed forever and that Russia has 'embarked on a path of evil' - comparing the Russian attack to Hitler's forces in World War Two. But he vowed to fight back, saying the military has already inflicted 'serious losses' on Russia.

He called on all Ukrainian citizens willing to defend their homeland to step forward, saying guns will be issued to everyone who wants one. He also asked for civilians to give blood to help wounded troops. And he asked world leaders to impose the 'harshest sanctions possible' on Putin.

British Prime Minister Boris Johnson, addressing the nation at midday, said western allies are preparing a 'massive' package of sanctions against Russia and told the people of Ukraine: 'We cannot and will not just look away.' Johnson referred to Putin as a 'dictator' who would never 'subdue the national feeling of the Ukrainians'.

It came after the Russian strongman gave an extraordinary address to the Russian nation - broadcast in the early hours during a UN meeting aimed at avoiding war - in which he declared a 'special military operation' to 'de-militarise' and 'de-Nazify' Ukraine in what amounted to a outright declaration of war. The video appeared to have been pre-recorded, around the same time as Putin's Monday address recognising Donbass as independent.

Putin also issued a chilling warning to any country thinking of coming to Ukraine's aid, vowing 'consequences greater than any you have faced in history'. 'I hope I have been heard,' he said.