Zelenskyy said Ukraine told no one about the Kursk invasion because its allies would have called it 'unrealistic'

• Ukraine didn't inform its allies about its planned incursion on Russia's Kursk region.
• The country's leader, Volodymyr Zelenskyy, said allies would have called the invasion "unrealistic."
• Ukraine ended up notching significant battlefield gains after catching the Russians off guard.

Ukrainian President Volodymyr Zelenskyy says his country kept its allies in the dark about its plans to attack Russia's Kursk region because they "would have said that it was unrealistic."

"This is why nobody had been informed about our preparations," Zelenskyy told the heads of Ukraine's foreign diplomatic missions in a speech delivered on Monday, according to a translation by Ukrainian media outlet Pravda .

"Now the real success speaks for itself: our active defensive actions on the other side of the border and Putin's inability to protect his territory from our defensive actions of this kind are very telling," he continued, referencing Russian leader Vladimir Putin .

Ukraine's allies weren't the only ones who didn't know about the attack. The planned invasion was kept so well under wraps that even Ukraine's own soldiers weren't informed until the last minute.

"We joked that it wasn't April 1st. The commander just smiled, knowing we had no idea what was awaiting us," a Ukrainian soldier identified only by his first name, Serhiy, said of the orders to invade Russia, The Economist reported on Sunday.

Russian forces were caught off guard when Ukraine mounted a shock offensive into the Kursk region on August 6. The surprise attack resulted in significant gains for Ukraine on the battlefield.

Oleksandr Syrskyi, the commander in chief of Ukraine's military, said last week that Ukraine had captured nearly 400 square miles of Russian territory in just a few days. That's close to the amount of Ukrainian territory that was seized by the Russians this year.

"When our Ukrainian defenders act like this, decisively and bravely, and when the operation is indeed well-prepared, Putin has no choice. And now the world sees that it is realistic, that it really works," Zelenskyy said on Monday, per Pravda.

Representatives for Ukraine's foreign ministry didn't immediately respond to a request for comment from Business Insider sent outside regular business hours.