There is a "high likelihood" that Kyiv's main children's hospital took a direct hit from a Russian missile during a series of airstrikes on Ukrainian cities, a U.N. rights mission said on Tuesday as the Kremlin continued to deny involvement.

Ukraine flew its flags at half mast in a national day of mourning to mark the deaths of 44 people across the country from Monday's air attacks, including four children and two people at the Ohmatdyt Children's Hospital in the capital.

"Analysis of the video footage and an assessment made at the incident site indicates a high likelihood that the children's hospital suffered a direct hit rather than receiving damage due to an intercepted weapon system," said Danielle Bell, head of the U.N. Human Rights Monitoring Mission in Ukraine.