Footage circulated by President Volodymyr Zelenskiy showed a huge blaze raging in a sprawling building and dark smoke billowing into the sky as onlookers stood outside. Reuters could not immediately verify the footage.

Zelenskiy said more than 1,000 people were in the shopping centre at the time of the attack. He gave no details of casualties but said: "It is impossible to even imagine the number of victims."

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

"It's useless to hope for decency and humanity from Russia," Zelenskiy wrote on the Telegram messaging app.

Kremenchuk, an industrial city of 217,000 before Russia's Feb. 24 invasion of Ukraine, lies on the Dnipro river in the region of Poltava and is the site of Ukraine's biggest oil refinery.

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

(Reporting by Pavel Polityuk and Max Hunder, Writing by Tom Balmforth, Editing by Timothy Heritage)