At least 20 soldiers were reported to have been killed in the attack, which local media said took place on Friday as a brigade gathered to receive awards in Ukraine's southern Zaporizhzhia region.

"This is a tragedy that could have been avoided," Ukrainian President Volodymyr Zelensky said in an evening address Sunday.

"A criminal investigation has been registered into the tragedy," he added.

AFP was not able to immediately verify the circumstances of the strike or the number of people killed.

The Ukrainian army confirmed on Saturday that a number of soldiers from its 128th Mountain Assault Brigade had been killed in a missile strike the day before, but did not provide casualty figures.

"(Russia) fired an Iskander-M missile at the personnel of the 128th Mountain Assault Brigade, killing the soldiers and causing injuries of varying severity to local residents," the army said.

One Ukrainian soldier said on social media that 22 people from the brigade had been killed, criticising commanders for having held the ceremony.

"Everyone is writing that 'Heroes died'. Although it is more appropriate to write 'Heroes became victims'," soldier Ivan Savytskyy said.

"They became victims of military rudimentary traditionalism in its worst form," he said.

Russia said in a defence ministry briefing Saturday that it had inflicted a "fire defeat" on a Ukrainian assault unit in Zaporizhzhia and that up to 30 people had been killed.

Ukraine's western Zakarpattia region, where the assault brigade is based, will observe a three-day mourning period starting Monday, local governor Victor Mykyta said.

"Our heroes are alive as long as the memory of them and their deeds lives on," he said Sunday.