At least 51 people were killed and 235 wounded when Russia struck a military institute in Ukraine's central town of Poltava with two ballistic missiles on Tuesday, the war's deadliest single attack this year.

Ukrainian President Volodymyr Zelenskyy said in a video that Russian forces struck with two ballistic missiles, damaging a building of the Military Institute of Communications. He ordered a full and prompt investigation.

Photographs posted on social media in Ukraine showed several bodies lying on the ground covered in dust and debris, with the badly damaged side of a large building behind them. Reuters could not immediately verify the images.

A further 15 people may still be under the rubble, according to Poltava regional Gov. Filip Pronin. Pronin said that many residents had donated blood for the injured in Poltava, located some 300 kilometres southeast of Kyiv and 120 km to the nearest Russian border. Local authorities announced three days of mourning.

"Every 15-20 minutes there are 'minutes of silence' to listen out for people who are under the rubble," Oleksandr Khorunzhyi, press officer of the emergency services, said in televised comments.

Some Ukrainians left worried messages on the institute's Facebook page seeking information about their loved ones.

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

"Thanks to the co-ordinated work of rescuers and medics, 25 people were rescued, 11 of whom were taken from the rubble. The rescuers are currently continuing their work," the ministry said.

Intense fighting in recent days

Russia had yet to comment on the attack.

Russia has intensified its missile and drone attacks on Ukraine 2½ years into the full-scale war.

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Last week, Ukraine was pummelled with the heaviest bombardment to date, and on Monday ballistic and cruise missiles targeted Kyiv, causing loud explosions.

Zelenskyy 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. He said Tuesday on Telegram that he had asked Canadian Prime Minister Justin Trudeau to step up advocacy for Ukraine's Western partners to allow strikes on military targets deep inside Russia, during a phone call between the leaders.

Fighting has intensified over the past month, with Russian forces advancing in heavy battles in eastern Ukraine, while Ukrainian troops have mounted their first large-scale cross-border assault into a Russian region, for which Moscow has vowed to retaliate.

Ukraine targeted Russia with more than 158 drones on the weekend, damaging an oil refinery near Moscow as well as a power station.