At least 51 people were killed and hundreds injured in a Russian missile attack on the Ukrainian city of Poltava Tuesday, authorities said, in one of the single deadliest strikes of the two-and-a-half-year war.

The attack partially destroyed a building used by the Poltava Military Institute of Communications, Ukrainian President Volodymyr Zelenskyy said.

“People found themselves under the rubble. Many were saved,” Zelenskyy said in a video posted on his Telegram channel. He said he has ordered “a full and prompt investigation.”

Shattered bricks could be seen inside the closed gates of the institution, which was off-limits to the media, and pools of blood were visible just outside.

Hours after the missile strikes, the smell of smoke had spread through town, which is about 350 kilometers southeast of the capital of Kyiv. Roads were covered with glass shards from shattered windows.

Ukraine’s Defense Ministry said the missiles hit shortly after an air-raid alert sounded, when many people were on their way to a bomb shelter. It described the attack as “barbaric.”

The defense agency said rescue crews and medics saved 25 people, including 11 who were dug out of the rubble.

Poltava Governor Filip Pronin announced three days of mourning, starting Wednesday.

“A great tragedy for Poltava region and entire Ukraine,” Pronin wrote on his Telegram page. “The enemy certainly must answer for all [its] crimes against humanity.”

The strike came as Russian President Vladimir Putin visited Mongolia. There was no indication that his hosts would heed demands to arrest him on an international warrant for alleged war crimes.

Zelenskyy appealed again for Ukraine’s Western partners to ensure swift delivery of military aid. He has previously chided the U.S. and European countries for being slow to make good on their pledges of help.

He also wants Ukraine’s allies to ease restrictions on what Ukraine can target inside Russia with the weapons they provide. Some countries fear that hitting Russia could escalate the war.

“Ukraine needs air defense systems and missiles now, not sitting in storage,” Zelenskyy wrote in English on Telegram. “Long-range strikes that can protect us from Russian terror are needed now, not later. Every day of delay, unfortunately, means more lost lives.”

Elsewhere, officials in Ukraine’s Zaporizhzhia region said a Russian strike on a hotel killed two people and injured two others.

Russian forces attacked areas across central and eastern Ukraine with four missiles and 35 drones, with Ukrainian air defenses shooting down 27 of the drones, Ukraine’s military said.

The Ukrainian air force said air defenses shot down the drones over the Chernihiv, Kharkiv, Kherson, Kyiv, Mykolaiv, Odesa, Poltava and Sumy regions.

Chernihiv Governor Viacheslav Chaus said on Telegram that falling debris damaged several buildings on the outskirts of the city of Chernihiv and injured two people.

Oleh Syniehubov, governor of the Kharkiv region, said on Telegram that Russia’s aerial attack damaged a house and a transportation facility.

Ukraine’s state railway reported a Russian drone struck a locomotive in Sumy.

Russia’s defense ministry said it shot down three Ukrainian drones over the Bryansk, Kursk and Kaluga regions.

Some information for this report came from The Associated Press and Reuters.