Ukrainian President Volodymyr Zelenskyy ordered late Thursday mobilization of all conscripts and reservists in all regions of the country amid Russian invasion.

The move will provide private vehicles, buildings, structures and land for use by the Ukrainian government and military for 90 days.

Ukraine has lost control of the Chernobyl nuclear site after a fierce battle with Russian troops, the Ukrainian military and an adviser to the presidential office said earlier.

Parts of the southern Kherson region and Hostomel airfield, northwest of the capital of Kyiv, were also reported to have been captured by the Russian forces.

Russian troops took over the Chernobyl while Ukrainian forces battled them on three sides on Thursday after Moscow mounted an assault by land, sea and air in the biggest attack on a European state since World War II.

The Russian military launched the much-anticipated invasion of Ukraine earlier on the same day, as President Vladimir Putin ignored international condemnation and sanctions and threatened other countries that any attempt to interfere would lead to "consequences you have never seen."

Big explosions were heard before dawn in Kyiv, Kharkiv and Odessa as world leaders decried the start of a Russian invasion that could cause massive casualties and topple Ukraine’s democratically elected government. Russian military columns crossed the border into Ukraine toward Chernihiv, Kharkiv and Luhansk regions, the Ukrainian border guard service said, in what appears to be an assault along the entire border with Russia.

Seven soldiers died in Russian airstrikes in the first few hours of the invasion, Ukraine’s Interior Ministry said.

The head of the Ukrainian military said Thursday he had received orders from President Volodymyr Zelenskyy to repel the Russian invasion of his country.

"The supreme commander the Armed Forces of Ukraine gave orders to inflict maximum losses against the aggressor," Commander-in-Chief of the Armed Forces of Ukraine, Major General Valeriy Zaluzhny said.

Earlier on Thursday, Zelenskyy introduced martial law, saying Russia has targeted Ukraine’s military infrastructure and explosions are heard across the country. Zelenskyy said he had just spoken to President Joe Biden and the United States was rallying international support for Ukraine. He urged Ukrainians to stay home and not to panic.