The broad offensive, which Kyiv said had already killed dozens of soldiers and several civilians, plunged Europe into one of its gravest security crises since World War II.

The first blasts rang out just minutes after Russian President Vladimir Putin gave a televised speech Wednesday evening saying that he was authorizing military action. He warned other countries that if they tried to intervene they would face a Russian response “so severe that no foreign nations have ever experienced it before.”

Battles and military strikes unfolded across the country throughout the day, Ukrainian officials said. Early in the day, they reported that cruise or ballistic missiles targeted military control centers in the area of the capital, Kyiv. Air raid sirens rang out across the city, and explosions were seen and heard there and in other cities, NBC News reporters on the ground said.

Later, Ukrainian President Volodymyr Zelenskyy said Russian forces were trying to seize the Chernobyl nuclear power plant north of Kyiv, near the border of Belarus. 

"Our defenders are giving their lives so that the tragedy of 1986 will not be repeated,” Zelenskyy tweeted, referring to the plant's reactor explosion, which caused the release of large amounts of radiation into the environment. “This is a declaration of war against the whole of Europe.”

An adviser to Ukraine’s interior minister warned that a strike on the nuclear waste storage facility could release radioactive dust that could blanket Ukraine, Belarus and European Union countries.

In a speech broadcast Thursday morning, Zelenskyy called on Ukrainians to take up arms.

“We are being attacked from the south, north, east and from the air,” he said. “We are giving away weapons and we will continue doing so to anyone who will ask for it in order to protect our sovereignty. Our future depends on each and every citizen.”

More than 40 Ukrainian soldiers had been killed, according to presidential adviser Oleksiy Arestovych. Another five people died when a Ukrainian military plane crashed and caught fire in the Kyiv region, the state emergency service said.

In the Donetsk region, a Russian shell hit a hospital, killing four and injuring 10, including six doctors, the Ministry of the Interior said in a Telegram chat. Earlier, at least six people died and seven were wounded in a bombing in the southern Odessa region, Ukraine’s General Office of the National Police said.

Ukraine said the Russian military was attacking with the help of Belarus, where Russian troops have been deployed for months. Surveillance video broadcast online by the Ukraine’s Border Guard Service showed tanks and armored vehicles crossing into Ukraine from Belarus. Four ballistic missiles were fired from there in a south-western direction, according to the government.

Battles also took place in Hostomel, home to an international cargo airport and located just outside Kyiv, as well as in the areas around Kharkiv in the east and Kherson in the south, Ukrainian officials said.

The government also reported “mass cyberattacks” on its websites.

NBC News was not immediately able to confirm the government reports of casualties, battles and troop movements.