At least 21 people were killed on Sunday morning in a Russian ballistic missile attack on the northeastern Ukrainian city of Sumy, Ukraine's Prosecutor General's Office said in a statement.

The attack also left other 34 people injured, including five children, the statement added.

The Sumy Regional Military Administration said that two ballistic missiles launched by Russian forces struck the city center, damaging residential and non-residential buildings.

Rescue operations were still underway as of midday local time (0900 GMT).

Ukrainian President Volodymyr Zelensky urged the international community to deliver a "firm" response to the attack.