Video footage appears to show a convoy of Russian tanks crossing the border into Ukraine from both Belarus and Crimea.
A clip broadcast on CNN showed the “surreal” sight of Russian tanks moving past the Ukrainian border guard and heading into the country.
“CNN now showing footage of Russian tanks crossing into Ukraine from Belarus. Just surreal,” wrote Financial Times journalist Max Seddon.
Similar footage, apparently from Ukrainian’s border guards at the crossing from Crimea - which Russia annexed from Ukraine in 2014 - shows numerous army lorries and tanks of various sizes, heading into the eastern European country.
Another video showed Russian troops on the outskirts of Ukraine’s second largest city, Kharkiv.
It came just hours after Vladimir Putin announced a "military operation" in Ukraine’s eastern Donbas region during a television address at 5.55am Moscow time (2.55am GMT), with the Russian president warning Moscow’s response will be "instant" should anyone try to take on Russia.
There have been subsequent reports of missile strikes and explosions in major Ukrainian cities, including Kiev.
Western leaders have expressed horror at the Kremlin’s actions.
European Commission president Ursula von der Leyen condemned Vladimir Putin for "bringing war back to Europe,” while US president Joe Biden said the war would bring with it "catastrophic loss of life" and the UK’s prime minister Boris Johnson said he was "appalled" by the "unprovoked attack.”
In Ukraine, citizens are trying to flee the country, while those who stay are being encouraged to join the country’s territorial defence units.
