Ukraine has experienced one of the deadliest attacks of the war after a Russian missile strike on the central city of Poltava hit a military training institute and a nearby hospital, killing at least 51 people and injuring more than 200 others.

The Ukrainian president, Volodymyr Zelenskiy, said in a video address that, according to preliminary information, two ballistic missiles had “partially destroyed” one of the buildings of the Poltava Military Institute of Communications, leaving people under the rubble.

Ukraine’s first lady, Olena Zelenska, called it “a stunning tragedy for all of Ukraine”, in a post on X. “The enemy hit an educational institution and a hospital,” she wrote. Some left worried messages on the institute’s Facebook page seeking information about their loved ones.

Zelenskiy said he held Russia accountable but had ordered a “full prompt investigation into all the circumstances of what happened”.

The strike has triggered anger on Ukrainian social media after unconfirmed reports said it had targeted an outdoor military ceremony, or roll call, with many blaming officials who allowed the event to take place despite the threat of Russian attacks.

Ukraine’s land forces confirmed that service personnel were killed in the strike and said an investigation was under way to establish whether enough was done to protect those in the facility. It said measures would be taken to prevent a recurrence.

Later on Tuesday it was announced that four Ukrainian cabinet ministers had resigned, ahead of an expected government reshuffle.

The deputy prime minister for European affairs, Olga Stefanishyna; the minister of strategic industries, Oleksandr Kamyshin (who has played a prominent role in boosting arms production); the minister of justice, Denys Maliuska; and the minister of the environment, Ruslan Strilets, all submitted their resignations

The US national security council spokesperson, John Kirby, condemned Tuesday’s attack as “another horrific reminder of Putin’s brutality towards the people in Ukraine” as the White House vowed that more aid would be dispatched to Ukraine in the coming weeks.

Poltava’s governor, Philip Pronin, said his administration could not provide more details of the circumstances of the strike “for security reasons”.

He added: “The enemy is using any means to bring Ukraine more pain and disorientate Ukrainians. Please trust only reliable sources,” he said.

Maria Bezugla, an MP who regularly criticises the country’s military leadership, accused high-ranking officials of endangering soldiers by allowing such events. “These tragedies keep repeating themselves. When will it stop?” she wrote on Telegram.

Poltava is about 200 miles (320km) south-east of Kyiv, far from the frontlines. Photographs posted on social media in Ukraine showed bodies, some in uniforms, lying on the ground covered in dust and debris. Substantial damage could be seen on two separate nearby multistorey buildings, with at least five floors exposed in one of them where the external wall had been blown off.

With fears that others remain buried under the debris, Oleksandr Khorunzhyi, the press officer of the emergency services, said in televised comments: “Every 15-20 minutes there are ‘minutes of silence’ to listen out for people who are under the rubble.”

A statement from Ukraine’s defence ministry said the “time between the air raid siren and the incoming deadly missile was so short that it caught people at the moment they were evacuating to the shelter”.

It added that rescue crews and medics had saved 25 people at the scene, including 11 who were dug out from the rubble.

Although the identities of the victims were not immediately disclosed, Serhiy Beskrestnov, a prominent Ukrainian Telegram blogger followed by many radio, communications and electronic warfare specialists in Ukraine’s military, posted a tribute to “my signals operator comrades”.

Russian Telegram channels described the site of the hit as a military training facility. It was not immediately clear how many of the victims were military or civilians.

Russia has struck civilian targets repeatedly throughout the two and a half years of full-scale war, and has intensified its air assault on Ukraine in recent weeks.

It launched a large missile and drone attack on Kyiv in the early hours of Monday, most of which was intercepted by Ukrainian air defence.

On Monday night, two people were killed in the city of Zaporizhzhia, according to the regional governor, including an eight-year-old boy.

Ukraine also targeted Russia with more than 158 drones at the weekend, damaging an oil refinery near Moscow and a power station, and last week Russia was pummelled with the heaviest bombardment to date.

Zelenskiy repeated his calls for more western air defences 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 defence 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.”

The deadly strikes came as Vladimir Putin received a red-carpet welcome to Mongolia on Tuesday. The country ignored calls to arrest him on an international warrant for alleged war crimes stemming from Moscow’s invasion of Ukraine.

The trip is the Russian president’s first to a member nation of the international criminal court (ICC) since it issued the warrant in March 2023. Before the visit, Ukraine urged Mongolia to hand Putin over to the court in The Hague, and the EU expressed concern that Mongolia might not execute the warrant.

The ICC has accused Putin of being responsible for the abductions of children from Ukraine. Member countries are required to detain suspects if a warrant has been issued, but Mongolia needs to maintain its ties with Russia and the court lacks a mechanism to enforce its warrants.

Putin was welcomed in the main square of the capital, Ulaanbaatar, by an honour guard dressed in vivid red and blue uniforms styled on those of the personal guard of the 13th-century ruler Genghis Khan, the founder of the Mongol empire.