Kyiv, Ukraine - Russia launched dozens of missiles at cities across Ukraine on Monday in an attack that killed at least 20 people and hit a children's hospital in Kyiv, officials said.

The rare day-time Russian barrage came as President Volodymyr Zelensky was due in Warsaw, the Polish government said, before he flies to the NATO summit in Washington.

Explosions rang out and black smoke could be seen rising from the center of Kyiv, AFP journalists reported.

Pictures distributed by officials from the children's medical facility in Kyiv showed crowds digging through mounds of rubble, black smoke billowing over a gutted building, and medical staff wearing blood-stained scrubs.

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"Russian terrorists once again massively attacked Ukraine with missiles. Different cities: Kyiv, Dnipro, Kryvyi Rig, Sloviansk, Kramatorsk," Zelensky said, listing major civilians hubs in the south and east of the country.

"More than 40 missiles of various types. Residential buildings, infrastructure and a children's hospital were damaged," he wrote on social media.

Zelensky said that there were an unknown number of people trapped under the rubble of the Okhmatdyt children's hospital and it was not immediately clear how many had been killed.

Municipal officials said earlier that at least seven people had been killed in the barrage that hit Kyiv.

In Zelensky's hometown Kryvyi Rig, which has been repeatedly targeted by Russian bombardments, the strikes killed at least 10 and wounded over 30, the mayor said.

"In Dnipro, a high-rise building and an enterprise were damaged. A service station was damaged. There are wounded," the Dnipropetrovsk governor Sergiy Lysak added.

In the eastern Donetsk region, where Russian forces have taken a string of villages in recent weeks, the regional governor said three people were killed in Pokrovsk – a town that had a pre-war population of around 60,000 people.

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There was no immediate comment on the strikes from the Kremlin.

"This shelling targeted civilians, hit infrastructure, and the whole world should see today the consequences of terror, which can only be responded to by force," the head of the presidential administration in Kyiv, Andriy Yermak, wrote on social media, following the attack.

Zelensky and other officials in Kyiv have been urging Ukraine's allies to send more air defenses, including US-made Patriot systems, to the war-battered country to help fend off fatal Russian aerial bombardments.

"Russia cannot claim ignorance of where its missiles are flying and must be held fully accountable for all its crimes," Zelensky said in another post on social media.