CNN: Military vehicles entering Ukraine from border crossing with Belarus

Russian forces have begun an attack on Ukraine, with reports of troops crossing the border to the north and south, missile strikes on the capital Kyiv and Russian President Vladimir Putin warning of bloodshed unless Ukrainian forces lay down their arms, CNN reports.

Reports of military action came as world leaders attempted to avert catastrophe with a meeting of the United Nations Security Council amid warnings that a possible invasion could start the biggest war in Europe since 1945.

The attack prompted immediate condemnation from the United States and its allies, which have threatened to enact "full scale" sanctions in response to Russian military aggression.

As the sound of air raid sirens rang out across the Ukrainian capital early Thursday, the country's President Volodymyr Zelensky moved to enact martial law and government ministers accused Moscow of launching a "full scale invasion." But as dawn broke in Kyiv, there was still traffic on the streets and in Kramatorsk a local official said people were calm and going to work.

Ukraine Foreign Minister Dmytro Kuleba tweeted on his official page that "peaceful Ukrainian cities are under strike" and called the situation "a war of aggression." And, an adviser for the Ukraine Interior Minister, Anton Gerashchenko, told journalists Russia's "invasion has begun" with "missile strikes on Kyiv," the capital.

Gerashchenko said troops had landed in the city of Odessa and were crossing the border in Kharkiv, the second-largest city in Ukraine located in the country's northeast. In a post on his Facebook page, he also said control centers such as airfields and military headquarters were being hit by shelling in Kyiv and Kharkiv, and that there was artillery fire along the border.

CNN witnessed, through a livestream video, troops atop a column of military vehicles entering Ukraine from a border crossing with Belarus. The livestream video was taken at the Senkivka, Urkaine crossing with Veselovka, Belarus. The column was seen entering Ukraine around 6:48 a.m. local time.