Moscow and Kyiv CNN  — 

Russian forces have begun an attack on Ukraine, with reports of troops crossing the border to the north and south, explosions in multiple cities including the capital Kyiv and warnings from Russian President Vladimir Putin of bloodshed unless Ukrainian forces lay down their arms.

The offensive that Russia called a “special military operation” 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 first blasts were heard at around 5 a.m. Thursday, prompting immediate condemnation from the United States and its allies, which have threatened to enact “full scale” sanctions in response to Russian military aggression.

As air raid sirens rang out across Kyiv early Thursday, Ukrainian President Volodymyr Zelensky moved to enact martial law and government ministers accused Moscow of launching a “full scale invasion.” As dawn broke, heavy traffic could be seen clogging roads heading west out of the city, while further east, near the Russian border, the mayor of Kharkiv urged citizens not to leave their homes.

Inhabitants of Kyiv leave the city following pre-offensive missile strikes of the Russian armed forces and Belarus on February 24.

Ukraine’s 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 to the Ukrainian Interior Minister, Anton Gerashchenko, told journalists Russia’s “invasion has begun” with “missile strikes on Kyiv.”

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, Ukraine crossing with Veselovka, Belarus. The column was seen entering Ukraine at about 6:48 a.m. local time.

Military vehicles are seen on a livestream video entering Ukraine from Belarus.

In recent weeks, Russia has amassed a significant number of troops, vehicles and tanks in Belarus near the border with Ukraine. During that time, the two countries have held joint military exercises across the countries, and near the Belarus-Ukrainian border.

CNN teams in Russia and Ukraine also heard blasts from several parts of Ukraine, including near the capital Kyiv and the port city of Odessa, and images released by the office of President Zelensky showed large explosions to the east of the capital Kyiv with huge columns of smoke rising into the air.

The General Staff of the Ukrainian Armed Forces said five Russian aircraft and a helicopter were shot down early Thursday, claims Russia denied via state news agency TASS.

An explosion is seen in the Ukrainian capital of Kyiv early Thursday, February 24.