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

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

"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.

It was the biggest bombardment of Kyiv in several months. The daylight attacks included Kinzhal hypersonic missiles, one of the most advanced Russian weapons, the Ukrainian Air Force said. The Kinzhal flies at 10 times the speed of sound, making it hard to intercept. City buildings shook from the blasts.

The attack struck Okhmatdyt children’s hospital in Kyiv, Ukraine’s biggest children’s medical facility. There was no immediate word on casualties there.

Russian forces launched multiple ballistic and cruise missiles against Ukrainian targets on Monday, Ukraine’s air force said, with explosions felt and heard across the capital, Kyiv.

The daylight attack included Kinzhal hypersonic missiles, one of the most advanced Russian weapons, the air force said. The Kinzhal flies at 10 times the speed of sound, making it hard to intercept. City buildings shook from the blasts.

The Kyiv city administration reported falling debris, presumably from intercepted missiles, in the Solomianskyi, Dniprovskyi, Holosiivskyi and Shevchenkivskyi districts of Kyiv, starting fires. Thick plumes of smoke rose from several Kyiv neighbourhoods.

The head of Ukraine’s presidential office, Andrii Yermal, said the attack occurred at a time when many people were in the city’s streets.

The attack came on the eve of a three-day NATO summit in Washington, which will look at how to reassure Ukraine of the alliance’s unwavering support and offer Ukrainians hope that their country can come through Europe’s biggest conflict since World War II.

Explosions were also reported by local officials in Ukraine’s central Dnipropetrovsk region.