"The mall is on fire, firefighters are trying to extinguish the fire, the number of victims is impossible to imagine," said the country's leader, writing on the Telegram messaging app.

Footage on social media showed a large number of people stood outside as a huge blaze raged inside the building in the city of Kremenchuk and dark smoke billowed into the sky.

The local mayor Vitaliy Meletskiy said the strike on the "very crowded" structure had caused deaths and injuries. He also said the site had no links to the armed forces.

Dmytro Lunin, the governor of the Poltava region, insisted there was no military target nearby which Russia could have been aiming at. He said at least 13 people were killed and 40 injured by two missiles.

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A rescue operation was under way and nine of the wounded were in a serious condition, said Kyrylo Tymoshenko, the deputy head of the Ukrainian presidential office.

Kremenchuk, which is the site of Ukraine's biggest oil refinery, lies on the Dnipro river in the region of Poltava.

Mr Zelenskyy stressed the shopping centre target presented "no threat to the Russian army" and had "no strategic value".

He accused Russia of sabotaging "people's attempts to live a normal life, which make the occupiers so angry".

"Russia continues to take out its impotence on ordinary civilians. It is useless to hope for decency and humanity on its part," the president said.

Boris Johnson has condemned President Vladimir Putin's "cruelty and barbarism" following the attack.

Firefighters and soldiers pulled out mangled pieces of metal as they searched for survivors.

"We don't understand how many people could be remaining under the rubble," the regional rescue service chief said.

There was no immediate comment from Russia, which denies deliberately targeting civilians.

The industrial city had a population of 217,000 before Russia's invasion in February.

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The attack came as Russia launched an all-out assault on the last Ukrainian stronghold in the eastern Luhansk region, "pouring fire" on the city of Lysychansk from the ground and air, according to the local governor.

Western leaders meeting at the G7 summit in Germany have pledged continued support for Kyiv, including more air defence systems, and further sanctions against Moscow.