A children's hospital has been seriously damaged in the Ukrainian capital Kyiv by Russian strikes, as 31 people were killed across the country in an early morning barrage.

At least 17 people were killed in Kyiv by the rare daytime attack, including two who died at the Ohmatdyt Children's Hospital - Ukraine's biggest paediatrics facility - which sustained major damage during the blast.

The head of the military administration in the central Ukrainian city of Kryvyi Rih said at least 10 people had been killed there, with three more killed in the eastern town of Pokrovsk and one in Dnipro.

Ukrainian President Volodymyr Zelensky, who is visiting Poland where he signed a security pact, vowed to retaliate.

"Different cities: Kyiv, Dnipro, Kryvyi Rih, Sloviansk, Kramatorsk. More than 40 missiles of various types. Residential buildings, infrastructure and a children's hospital were damaged," Mr Zelensky wrote earlier on social media.

Lesia Lysytsia, a doctor at the children's hospital, told the BBC the moment the missile struck was "like in a film" with a "big light, then an awful sound".

"One part of the hospital was destroyed and there was a fire in another. It's really very damaged - maybe 60-70% of the hospital," she said.

Pictures from the scene showed young children - some with IV drips - sitting outside the hospital as it was evacuated.

Dr Lysytsia said Ohmatdyt is a big hospital which carries out many functions, including cancer treatment and organ transplants.

"Now we are in the process of evacuating patients to the nearest hospital.. [but] many patients are intubated and on ventilators and cannot have contact with other patients or go outside," she said.

Hospital officials told Ukrainian TV that about 20 children were being treated from the ward which was hit.

In the hours after the initial strike another explosion rocked the area nearby and hundreds of people took shelter in an underpass.

Mr Zelensky said in a social media post that people were trapped under the rubble of the hospital.

"Right now, everyone is helping to clear the rubble - doctors and ordinary people," the president wrote.

Vitaliy Klitschko, Kyiv's mayor, told the Reuters news agency from the hospital that the bombardment was "one of the the worst attacks" on the capital since the start of the war.

The State Emergency Service of Ukraine posted on social media that a separate maternity unit in Kyiv had also been partially destroyed by falling debris, killing four people and wounding three.

Ukrainian tennis player Elina Svitolina will wear a black ribbon when she plays her round of 16 match at Wimbledon on Monday afternoon to commemorate the victims, the Women's Tennis Association said.

Elsewhere, Oleksandr Vilkul, head of the military administration in Kryvy Rih, posted on the Telegram messaging app that at least 10 people were killed and 31 wounded following the attack on the city. Of those, he said 10 were seriously injured.

Kryvy Rih is Mr Zelensky's home town and has been repeatedly attacked by Russia since it launched its full-scale invasion of Ukraine in February 2022.

Dnipro regional head Sergiy Lysak said one person was killed in Dnipro city and six more injured. He added that a high-rise building and a business had been hit.

Three people were killed in Pokrovsk, in the eastern Donetsk region, where Russian forces have taken control of a number of villages in recent weeks.

The Russian bombardment comes as Indian Prime Minister Narendra Modi arrived in Moscow for a two-day state visit and is due to hold talks on Monday with President Vladimir Putin at the Kremlin.

Russia denied targeting civilian infrastructure, insisting the strikes had been aimed at military facilities.

Ukraine's Defence Minister Rustem Umerov responded to the attacks by urging the country's allies to help quickly strengthen its air defences.

"Our defence capabilities are still insufficient... We need more air defence systems," he said.

The UN's human rights monitoring mission in Ukraine has said civilian casualties have been mounting in recent months, as Russia renewed its air campaign. A recent report said May was the deadliest month for civilian deaths in almost a year.