“Dear warriors, the main battles are coming soon. We must free our land and our people from Russian slavery,” Zelenskyy said.
He delivered the remarks at an unidentified military base, surrounded by ally-supplied military vehicles and members of the Border Guard's newly formed elite ‘Steel Border’ Assault Brigade.
There has long been talk of a Ukrainian spring counter-offensive against invading Russian forces, but it has yet to be launched.
Also on Sunday the governor of a Russian region bordering Ukraine said four people were killed in a Ukrainian rocket attack. The rockets hit homes in the village of Suzemka, nine kilometre from the Ukrainian border, said Bryansk regional governor Alexander Bogomaz.
He said two other residents were injured and that defence systems had knocked down some of the incoming shells.
Bryansk and the neighbouring Belgorod region have experienced sporadic cross-border shelling throughout the war. In March, two people were reported killed in what officials said was an incursion by Ukrainian saboteurs in the Bryansk region.
Also Sunday, Gov. Oleksandr Prokudin said his Kherson region in Ukraine came under Russian artillery fire 27 times in the past 24 hours, killing one civilian.
An expected spring counteroffensive by Ukraine could be concentrated in the Kherson region, a gateway to Crimea and other Russian-occupied territory in the southern Ukrainian mainland. Ukrainian forces forces drove Russian forces out of the regional capital Kherson last year, a significant defeat for Russia.
Ukrainian President Volodymy Zelenskyy said the counteroffensive wouldn't wait for the delivery of all promised military equipment.
“I would have really wanted to wait for everything that was promised," Zelenskyy told Finnish, Swedish, Danish, and Norwegian journalists. "But it happens that the terms (of weapons deliveries and counteroffensive), unfortunately, do not coincide a little bit. And, I will say frankly, we pay attention to the weather.”
Ukraine is particularly hopeful that it will receive Western fighter jets, but Zelenskyy said his forces won't delay the counteroffensive for that, so as not to "reassure Russia that we still have a few months to train on the planes, and only then will we start.”
The head of Russia's Wagner mercenary group that is leading his country's battle in the eastern Ukrainian city of Bakhmut gave an even more precise timetable for the Ukrainian counteroffensive.
The Ukrainian military will launch the counteroffensive by May 15 because by then strong rains will have stopped and soil will be dry enough for tanks and artillery to move, Wagner founder Yevgeny Prigozhin said in a video interview with a Russian journalist posted Saturday.
Prigozhin also said Wagner needs about 80,000 shells per day, but is only receiving about 800 a day, and that the choice was to either withdraw or die and fight if they more shells are not supplied.
The battle for Bakhmut is the biggest of the war.