Kyiv, Ukraine - Rescue workers in Kyiv were still clearing debris as Ukraine mourned those killed in a massive Russian attack that tore open a children's hospital.

President Volodymyr Zelensky said 38 people across Ukraine were killed – including four children – and 190 wounded in the wave of nearly 40 missiles that targeted several towns and cities.

Officials announced a day of mourning in the capital with flags to be hung at half-mast and entertainment events postponed, as cranes and first responders were working at the scene of the Okhmatdyt pediatric hospital.

Hundreds of volunteers gathered to help clear rubble and search for survivors.

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"The rescue operation after yesterday's Russian missile attack continues," Zelensky wrote on social media, adding that patients from Okhmatdyt had been transferred to other facilities.

"We continue our work to increase the protection of our cities and communities from Russian terror. There will be decisions. The world has the necessary strength for this," Zelensky added.

He has been urging allies to bolster Ukrainian air defense systems and was expected to renew those calls as a NATO summit kicked off later Tuesday in Washington.

The Ukrainian president also called for a UN Security Council meeting on the strikes that also claimed some 10 lives in his hometown of Kryvyi Rig. The US, meanwhile, denounced Russia's assault as "savage" and deliberate on Monday.

City officials told AFP on Tuesday that two people had been killed at the hospital – a doctor and a visitor, but that there could still be victims trapped under the rubble.

Ukraine said a Russian Kh-101 cruise missile had smashed into the renowned children's hospital, with Moscow responding only by blaming Ukrainian air defenses.