A missile strike on a mail depot in the eastern Ukrainian city of Kharkiv killed six people, Ukrainian officials said Sunday. A further 16 people were injured in the blast late Saturday, which is believed to have been caused by a Russian S-300 rocket, Kharkiv governor Oleh Syniehubov said on social media. All of the victims were employees of private Ukrainian postal and courier service Nova Poshta.

In a statement, the company said that the air raid siren had sounded just moments before the attack, leaving those inside the depot with no time to reach shelter. It announced that Sunday would be a day of mourning for the firm. Ukrainian President Volodymyr Zelenskyy described the strike as an attack on an “ordinary civilian object.”