Ukrainian President Volodymyr Zelensky said Thursday that any citizen who wanted to defend the country from Russian attacks would be provided with a weapon.

"We will give weapons to anyone who wants to defend the country. Be ready to support Ukraine in the squares of our cities," he tweeted as the conflict between both neighboring countries escalated.

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Russia unleashed airstrikes against Ukraine Thursday morning as residents took shelter in subway stations and buildings and others tried to flee cities by car. The conflict is possibly the largest on European soil since World War II and has drawn widespread international condemnation.

In a televised speech, Russian President Vladimir Putin said the goal of the military operation is for the "for the demilitarization and de-Nazification of Ukraine."

After the speech, Russian tanks crossed into the country from Belarus and Crimea, which was annexed by Moscow in 2014.

Ukraine's foreign ministry said Russia was launching attacks on "peaceful Ukrainian cities from different directions," including the Donbas region, which is recognized by Moscow as a breakaway state, and called on allies to provide the Eastern European nation with weapons and military equipment.

Ukrainian servicemen sit atop armored personnel carriers driving on a road in the Donetsk region, eastern Ukraine on Thursday. Russian President Vladimir Putin announced a military operation in Ukraine and warned other countries that any attempt to interfere with the Russian action would lead to "consequences you have never seen." (AP Photo/Vadim Ghirda)

A tank of Ukrainian forces moves as following Russia's military operation in Chuhuiv town, Kharkiv Oblast, Ukraine. (Photo by Wolfgang Schwan/Anadolu Agency via Getty Images)

Demonstrators hold placards and flags as they attend a protest outside the Russian Embassy, in London on Wednesday. (AP Photo/Alberto Pezzali) (AP Photo/Alberto Pezzali)

A wounded woman is seen as airstrike damages an apartment complex outside of Kharkiv, Ukraine on Thursday. (Photo by Wolfgang Schwan/Anadolu Agency via Getty Images)

Ukrainian Foreign Minister Dmytro Kuleba speaks during a joint press conference with Italian Foreign Minister following their meeting in Kyiv on Feb. 15. ( Sergey Dolzhenko/pool/AFP Via Getty Images)

Ukrainian servicemen stand by a destroyed house near the frontline village of Krymske, Luhansk region, in eastern Ukraine last weekend. (AP Photo/Vadim Ghirda) (AP Photo/Vadim Ghirda)

In response to the attacks, Zelensky declared martial law and cut diplomatic ties with Moscow.

"Don't panic. We are strong. We are ready for everything," he said in a video address to the country. "We will defeat everyone. Because we are Ukraine."

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On Thursday, President Biden imposed additional sanctions on Russia over Putin's aggression but acknowledged it would take time for the measures to set in.

"Putin is the aggressor. Putin chose this war. And now he and his country will bear the consequences," he said.

Ukraine president says he will give weapons to citizens who want to defend country

Ukrainian President Volodymyr Zelensky declared martial law as the country came under attack from Russian forces