Explosions have already been heard in Kyiv, Kharkiv and in the Belgorod province of Russia.
Explosions also rocked the breakaway eastern Ukrainian city of Donetsk and civilian aircraft were warned away as the United States said a major attack by Russia was imminent.
Putin made the declaration in a televised speech at the same time as the UN Security Council was imploring him to stop.
Putin says Russia does not plan to occupy Ukrainian territory.
He called on Ukrainian soldiers to immediately lay down their weapons and go home.
