Russian missiles and drones again crisscrossed Ukrainian skies on Monday night in strikes that killed more than 50 people and injured hundreds, Ukrainian officials said.
Most of the casualties were reported at an attack in Poltava, according to officials. Two ballistic missiles struck the Poltava Military Communications Institute as well as a nearby hospital, according to President Volodymyr Zelenskyy.
At least 51 people were killed and 217 injured in the attack in Poltava, according to Zelenskyy.
"I have ordered a full and prompt investigation into all the circumstances of what happened," Zelenskyy said on his official Telegram channel on Tuesday morning.
First lady Olena Zelenska described the attack as a "stunning tragedy for all of Ukraine."
One of the buildings of the military academy was partially destroyed in the strike, the Ukrainian Defense Ministry said.
"The time interval between the alarm and the arrival of the deadly rockets was so short that it caught people at the moment of evacuation to the bomb shelter," the Ukrainian Defense Ministry said in a post on Telegram.
Many were trapped under the rubble, and rescue workers are continuing to search for people, the ministry said.
"It is known that there are people under the rubble of the destroyed building," Zelenskyy said on Telegram. "Everything is being done to save as many lives as possible."
Valeriy Parkhomenko, the deputy mayor of Poltava, also told ABC News there are believed to be survivors under the rubble.
He said classes had been taking place in the military academy at the time of the strikes. There had been virtually no time for people to reach shelters, with the missiles hitting roughly two minutes after they were launched, he said.
A three-day mourning period has been declared in the city.
MORE: Russian missiles strike Kyiv, injuring 2, city officials sayIn total, Ukraine's air force said on Telegram that Russia fired three Iskander ballistic missiles from occupied Crimea, one Kh-59/69 air-launched missile from Russia's western Kursk region and 35 Iranian-made Shahed attack drones from two areas in Kursk and Crimea.
Ukrainian air defenses downed 27 drones, the air force said, with six more "lost."
Ukraine's Interior Ministry said two people -- a 38-year-old woman and her 8-year-old son -- were killed in a strike on a hotel complex in the southeastern city of Zaporizhzhia.
Two other members of the family -- the father and a 13-year-old girl -- were buried under rubble but later rescued. Both are hospitalized in "serious condition," the ministry said.
Further north, in the city of Dnipro, one person was killed and at least six injured by a Russian missile attack, the Interior Ministry wrote on Telegram.
Ukraine's air defense units were active overnight in Kyiv, Odesa, Kharkiv, Mykolaiv, Kherson, Poltava and the Chernihiv and Sumy regions, the air force said.
Russia's intensifying long-range attacks on Ukrainian military, infrastructure and civilian targets have prompted Kyiv to push its Western partners -- chief among them the U.S. -- for permission to use Western weapons against airfields and launch sites within Russian borders.
Ukraine has scored notable successes within Russia with its own domestically produced drones and missiles, but Zelenskyy has repeatedly said Kyiv needs more advanced capabilities.
"The terrorist state must feel what war is," the president said on Sunday. "To force Russia into peace, to move them from deceitful rhetoric about negotiations to taking steps to end the war, to clear our land of occupation and occupiers, we need effective tools."
Following a deadly Russian-guided bomb strike on the city of Kharkiv last week, Zelenskyy said in his nightly video address that such attacks can only be stopped "by striking Russian military airfields, their bases, and the logistics of Russian terror."
"We talk about this every day with our partners," he said. "We persuade. We present arguments."
Curtailing Russia's ability to strike from the air, Zelenskyy added, would be "a strong step to force Russia to seek an end to the war and a just peace."
In a statement Tuesday night, President Joe Biden called the Poltava attack "deplorable."
"I condemn this deplorable attack in the strongest possible terms," Biden said in his statement.
"This assault is a tragic reminder of Putin’s ongoing and outrageous attempts to break the will of a free people. But for two and a half years, the people of Ukraine have stood unbowed. And the United States will continue [to] stand with them—including providing the air defense systems and capabilities they need to protect their country," his statement continued, adding, "Make no mistake: Russia will not prevail in this war. The people of Ukraine will prevail. And on this tragic day, and every day, the United States stands with them."