"We must prepare ourselves for a long war in Ukraine," NATO secretary-general Jens Stoltenberg said in an interview with Germany's Funke media group published Sunday.
"Most wars last longer than expected when they first begin," he said.
"We are all wishing for a quick peace," Stoltenberg said, "but at the same time, we must recognize if [Ukrainian] President [Volodymyr] Zelenskyy and the Ukrainians stop fighting, their country will no longer exist."
Meanwhile, Russia targeted Ukraine's Odesa region early Sunday with a combined drone and missile attack, hitting an agricultural facility, Ukraine's Air Forces said on the Telegram messaging app.
The extent of the damage was not immediately clear.
Also Sunday, Ukraine launched a drone attack on Crimea and Moscow.
A regional Crimean official said a drone hit a fuel tank, causing a fire that was extinguished.
The drones over Moscow interfered with air traffic over the capital.
The British Defense Ministry said Sunday in its daily intelligence update on Russia's invasion of Ukraine that Russia is "likely" reinforcing its defenses around the occupied town of Tokmak in southern Ukraine.
The town is 16 kilometers from the front line and Tokmak is being outfitted to become a "lynchpin" of Russia's second main line of defenses.
Attention to the town's defenses "likely indicates Russia's growing concern about Ukrainian tactical penetrations of the main defensive line to the north."
Some information for this report came from Agence France-Presse and Reuters.