The missile strike partially destroyed a children’s hospital in the Ukrainian city.

"In Ukraine today, 37 people were killed, three of whom were children, and 170 were injured, including 13 children, as a result of Russia’s brutal missile strike," Volodymyr Zelenskyy wrote on X.  "A Russian missile struck the largest children's hospital in Ukraine, targeting young cancer patients. Many were buried under the rubble," he said.

'Slamming Russia over the missile strike, the Ukrainian leader said: "It is a huge disappointment and a devastating blow to peace efforts to see the leader of the world's largest democracy hug the world's most bloody criminal in Moscow on such a day."

On Tuesday, Ukraine is following a day of mourning to mark the loss of lives in the Russian missile strike.

Russia, however, denied striking Kyiv hospital.

US President Joe Biden reacted to the strike and said in a statement: "Russia’s missile strikes that today killed dozens of Ukrainian civilians and caused damage and casualties at Kyiv’s largest children’s hospital are a horrific reminder of Russia’s brutality. It is critical that the world continues to stand with Ukraine at this important moment and that we not ignore Russian aggression."

"Together with our allies, we will be announcing new measures to strengthen Ukraine’s air defences to help protect their cities and civilians from Russian strikes. The United States stands with the people of Ukraine," he said.

Condemning the daytime attacks, the UN’s top aid coordinator in Ukraine, Denise Brown, said that several cities were targeted, including the capital, Kyiv, Kryvyi Rih and Pokrovsk.

The attacks happened just “as people were starting their day. Dozens of people have been killed and injured,” said Brown, who reported heavy damage to the children’s hospital in the centre of Kyiv.

In a post on X, UN chief António Guterres strongly condemned the missile attacks while High Commissioner for Human Rights Volker Türk deplored the assault on what are densely populated areas.

Any strike on a children’s hospital is an “abominable” act, the rights chief said, adding that 'those with influence must do everything to ensure these attacks stop immediately'.

“It is unconscionable that children are killed and injured in this war. Under international humanitarian law, hospitals have special protection. Civilians must be protected,” Ms. Brown insisted.

The latest bloody development follows an alert from UN human rights office monitors that May saw the highest number of civilian casualties caused by Russian attacks in nearly a year.

According to the report by the UN Human Rights Monitoring Mission in Ukraine (HRMMU), between 1 March and 31 May, at least 436 civilians were killed and a further 1,760 injured. Casualties included six media workers, 26 healthcare employees, five aid workers and 28 emergency services workers.

The UN report added that the majority (91 per cent) of the casualties were in territory controlled by Ukraine and nine per cent in Russian-occupied territory.

In the same reporting period, the Russian authorities reported that 91 civilians were killed and 455 injured in Russia from attacks launched by Ukrainian armed forces, primarily in Belgorod, Briansk and Kursk regions.