Ukrainian President Volodymyr Zelenskyy declared martial law across the country as Russian forces began military operations early Thursday, NBC News reports.
Zelenskyy also spoke directly over the phone with U.S. President Joe Biden late Wednesday. Biden said in a statement shortly after the call that he told the Ukrainian president he "condemned this unprovoked and unjustified attack by Russian military forces."
Biden continued, "I briefed him on the steps we are taking to rally international condemnation, including tonight at the United Nations Security Council. He asked me to call on the leaders of the world to speak out clearly against President Putin’s flagrant aggression, and to stand with the people of Ukraine."
Ukraine's president has urged calm, and did not detail what martial law would mean for the country, where numerous explosions have been reported near major cities.
Later in the day, an official Telegram messaging app channel for Kyiv, Ukraine's capital, advised all non-essential workers to "stay at home and be prepared to go to the shelter if the sirens are turned on." Schools were also closed Thursday, the message said, while hospitals would work in an "intensified mode."
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Residents woke up to the news that Putin would begin military operations in eastern Ukraine, and before sunrise, the sounds of explosions and air raid sirens rang out.
“Today we need each and everyone of you to stay calm if you can stay at home,” Zelenskyy said. “We are working, the army is working, all sector of defense and security of Ukraine is working. I will keep in touch all the time as well as security and defense council. I will talk to you soon. Don’t panic, we’re strong, we’re prepared, we will win.”
Kyiv Mayor Vitali Klitschko, meanwhile, urged residents via the Telegram Thursday morning local time to remain calm as Russian forces descended on Ukraine.
"Dear Kyivans! Ukraine is attacked by the aggressor. The roar of shells can be heard in Kyiv. The worst enemy now is panic. We do not lose endurance," Klitschko said, while warning residents to prepare themselves to leave in their homes if needed.
This is a live update. Click here for complete coverage of the crisis in Ukraine.