Ukrainian Defense Minister Oleksii Reznikov said the country’s armed forces were “heroically” repelling the Russian invasion deeper into the country, warning that “tomorrow the Russians will learn the price of [Russian president Vladimir] Putin’s madness.

In a Facebook post late Thursday, Reznikov said, “Ukraine has been pushing back the Russian occupiers for more than 16 hours.”

“The armed forces, guards, border guards, rescuers and fighters from other security forces are defending themselves heroically. The entire nation is gradually joining the resistance. Thousands of citizens have already joined the Territorial Defense Forces,” he added.

Reznikov said “heavy fighting continues” north of the capital Kiev, where government troops were pushing back Russian forces from Hostomel airport, as well as in southern and eastern areas where government troops have fought Russian-controlled separatists since Moscow annexed Ukraine’s Crimean peninsula in 2014.

“The situation in the south of the country is not easy,” Reznikov said amid reports that Russian forces had captured large swathes of territory north of Crimea. Ukrainian forces said a strategic objective was to capture water channels to revive Crimea’s fresh water supply, which Ukraine cut off shortly after annexing the peninsula.

“The enemy did not expect such resistance. He was arrested near Kharkiv”, the country’s second largest city just a few dozen kilometers from Ukraine’s northeastern border with Russia.

“We actively use samples of modern weapons received from partners. . . tomorrow the Russians will know the price of Putin’s madness,” said Reznikov before adding: “we will not back down on our lands.”

Reznikov’s comments come as Ukrainian officials, including Oleksiy Arestovych, an adviser in President Volodymyr Zelensky’s office, said government troops ‘repelled’ Russian forces in a day-long battle over the airport of Hostomel in the north of Kyiv.

“The Russian landing team has been destroyed,” he added.

Reznikov urged Ukrainians to prepare for “a new wave of attacks, including airstrikes” overnight.