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

US President Joe Biden condemned the "deplorable attack", which Kyiv said hit a military training facility and a nearby hospital, though authorities did not say how many of the victims were military or civilians.

Ukrainian President Volodymyr Zelensky vowed to hold what he called "Russian scum" accountable, while rescuers worked to clear the rubble.

"According to the information available now, this Russian strike killed 51 people," Zelensky said in his evening address.

"The number of injured is 271. We know that there are people under the rubble of the destroyed building. Everything is being done to save as many lives as possible," Zelensky said.

Washington, Berlin and London all condemned the strike.

Biden vowed Washington would continue military aid to Kyiv, "including providing the air defense systems and capabilities they need to protect their country".

British Foreign Secretary David Lammy called the attack a "sickening act of aggression", while German Foreign Minister Annalena Baerbock said the brutality of Russian President Vladimir Putin "knows no limits".

Putin travelled to Russia's Vladivostok on Tuesday, the Kremlin said, after concluding a visit to Mongolia.

The trip was his first to an International Criminal Court (ICC) member since it issued a warrant for his arrest related to the war in Ukraine.

- 'Full investigation' -

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

Zelensky said he had ordered a "full and prompt investigation into all the circumstances".

Two Russian ballistic missiles hit the hospital and educational institution, partially destroying one of the buildings, Zelensky said.

The strike took place in the morning in Poltava, a city with a pre-war population of around 300,000 people, some 300 kilometres (190 miles) east of Kyiv.

"The window blew open. Dust was everywhere. I just had time to tell my sister that a rocket was flying," said Yevgeniya Chyrva, a resident in a building damaged during the attack.

The defence ministry said that the time between the alarm and the arrival of the missiles was "so short that it caught people in the middle of evacuating to the bomb shelter".

The Poltava military communications institute, founded in the 1960s when Ukraine was part of the USSR, specialises in training telecommunications specialists.

"One of the institute's buildings was partially destroyed, and many people were trapped under the rubble," the defence ministry said.

An AFP journalist on the scene saw several ambulances heading towards the affected site shortly after the attack on the military institute.

Rescuers were still at work after managing to save 25 people, including 11 trapped under the rubble, the defence ministry said.

- Government reshuffle -

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

Ukrainian MP Maria Bezugla, who regularly criticises the country's military leadership, accused high-ranking officials of endangering soldiers.

"These tragedies keep repeating themselves. When will it stop?" she posted on Telegram.

The attack came as Ukraine's leadership signalled a major government reshuffle was underway, with at least six officials submitting their resignation Tuesday.

The Minister for Strategic Industries, Minister for Justice and Minister of Environmental Protection were among those to step down.

Zelensky has ordered several reshuffles since the war began, sacking his defence minister last September after a series of corruption scandals and more recently replacing his top commander amid setbacks on the battlefield.

The move follows another recent scandal for Ukraine's army command after a US-made F-16 fighter jet crashed in combat last week, killing the pilot.

The crash of the F-16 was a high-profile setback for Kyiv, which had lobbied the West to send the advanced fighter jet for months, and triggered the dismissal of the country's air force chief, Mykola Oleshchuk.