Ukrainian President Volodymyr Zelenskyy and his family remain Russia's primary target in the military takeover of Ukraine, retired four-star Gen. David Petraeus told Fox News on Thursday.

"President Zelenskyy is quite courageous," Petraeus said on "Jesse Watters Primetime." "I thought that there might be a plan to evacuate him … he apparently is staying in the city even though as he announced today, he is target number 1 for the Russians and his family is target number 2."

Russia unleashed airstrikes against Ukraine on Thursday morning as residents took shelter in subway stations and buildings and others tried to flee cities by car. The conflict escalated, marking the largest attack on European soil since World War II.

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Zelenskyy, who declared martial law and cut diplomatic ties with Moscow, encouraged Ukrainians to remain steadfast, and reportedly offered to provide any citizen who wanted to defend the country from Russian attacks with a weapon. 

While Ukrainians have rhetorically "sounded determined," they proved themselves militarily determined on the battlefield Thursday, putting up an unexpected fight, Petraeus said.

"That very important fight for that airfield just outside of Kyiv where presumably the Russians would have put in more air assault infantry tonight, they will not be able to do it," Petraeus said.

"Reports of very heavy Ukrainian casualties indicate how serious the fighting has been," he said. "I have heard that they have, indeed, knocked down Russian aircraft. They have taken out Russian tanks. The idea that they would be able to roll to Kyiv in two days just is not going to be the case."

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Russia will continue its "attempt at shock and awe tonight, unquestionably," the former CIA chief predicted. 

"But I think what Russians are finding is that this is a vast country. As many have pointed out, it's a huge population. Its people are very determined not to come under the Russian boot."

Women cry after they entered Poland from Ukraine at Polish-Ukrainian border crossing after Russia bombarded Ukrainian territory. (Photo by Dominika Zarzycka/NurPhoto via Getty Images) (Photo by Dominika Zarzycka/NurPhoto via Getty Images)

The Stinger missiles and other weapons provided by the United States and NATO allies are credited with bringing down Russian military helicopters and planes on Thursday, Petraeus said.

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"Those are among the weapons that probably brought down the Russian helicopters and planes today," he said. "So, again, I suspect the Russians are not entirely pleased with how this first day has gone."

At the end of the day, "the Russians can absolutely overwhelm the Ukrainians," he said. 

"They are completely outmatched, but the idea that they would put up such a determined front today, I think, is really quite significant."