Columns of Russian tanks and armored vehicles are advancing on Ukraine’s capital Kyiv and have reached the outskirts of the city, after a second night of heavy Russian shelling and airstrikes. U.S. intelligence officials view the military offensive as a move to encircle Kyiv and topple the Ukrainian government — something they warn could happen within days. On Thursday, Russian President Vladimir Putin gathered Russia’s wealthiest men for a meeting just hours after Russian forces began a sweeping attack on Ukraine by land, air and sea. Putin said he had no choice but to attack in order to ensure Russia’s security.

President Vladimir Putin: “Everything that is happening is a desperate measure. They left us no other option. They’ve created such security risks that we couldn’t react differently.”

Ukrainian President Volodymyr Zelensky said he is remaining in the capital despite threats to his life. On Thursday, he vowed to defend Ukraine, while saying he is open to talks with Russia and discussing the issue of neutrality. Zelensky has ordered a full military mobilization, with all men aged 18 to 60 ordered to remain in Ukraine to face possible conscription into military service.

Elsewhere on Thursday, Russian forces captured the Chernobyl nuclear power plant after a fierce battle, raising fears of a new nuclear disaster. The White House expressed alarm over what it called credible reports that staff at the Cherobyl plant were being held hostage.