At least 41 people were killed and more than 180 wounded Tuesday when Russia hit a military institute in the central town of Poltava, Ukraine President Volodymyr Zelenskyy said.
It was one of the deadliest single strikes of the war.
Zelenskyy said in a video that Russian forces struck with two ballistic missiles, damaging a building of the Military Institute of Communications.
He added that he had ordered a full and prompt investigation into the circumstances of the attack.
"The Russian scum will definitely be held accountable for this strike," Zelenskyy said on the Telegram messaging app.
He repeated his calls for more Western air defenses and urged allies to allow their long-range weapons to be used for strikes deeper into Russian territory in order to protect Ukraine.
"We keep telling everyone in the world who has the power to stop this terror: air defense systems and missiles are needed in Ukraine, not in a warehouse somewhere.
"Long-range strikes that can protect us from Russian terror are needed now, not some time later. Unfortunately, every day of delay means loss of life."