The attack, in the central city of Kremenchuk far from any frontline, drew a wave of global condemnation, with France's Emmanuel Macron among leaders who called it a "war crime".
Ukraine said Russia had killed civilians deliberately.
Russia said it had struck a nearby arms depot and suggested the mall was empty.
At a summit in Germany, leaders of the G7 industrialised democracies announced plans for a price cap on Russian oil, designed to starve Russia of the resources for war without exacerbating a global economic crisis.
Next up will be a NATO summit in Spain, at which the military alliance is expected to announce hundreds of thousands of troops shifting to a higher state of alert and an overhaul of its strategic framework to describe Russia as an adversary.
Relatives of the missing in Kremenchuk were lined up at a hotel across the street from the wreckage of the shopping centre, where rescue workers had set up a base.
Weary firefighters sat on a kerb after a night battling the blaze and searching for survivors, mostly in vain.
Oleksandr, dousing his face from a water bottle, said his team had worked all night.
"We pulled out five bodies. We didn't find anybody alive," he said.
Ukrainian President Volodymyr Zelenskiy accused Russia of deliberately targeting civilians in "one of the most defiant terrorist attacks in European history".
Russia's defence ministry said its missiles had struck an arms depot storing foreign weapons, which exploded, causing the blaze that spread to the nearby mall.
Ukraine said there was no military target in the area.
"Russia's goal is for as many Ukrainians as possible to close their eyes forever, for the rest to stop resisting and submit to slavery," Andriy Yermak, chief of Ukraine's presidential staff, said on Twitter.
"That's the way the terrorist state acts."
Russia described the shopping centre as disused and empty.
But that was contradicted by the relatives of the dead and missing, and the dozens of wounded survivors such as Ludmyla Mykhailets, 43, who had been shopping there with her husband when the blast threw her into the air.
"I flew head first and splinters hit my body. The whole place was collapsing," she said at a hospital where she was being treated.
G7 leaders said the attack was "abominable".
Russian President Vladimir Putin and those responsible would be held to account, they said in a statement.
At the end of its summit, the G7 announced a new approach - leaving Russian oil on the market while imposing a cap on the price countries could pay for it.
"We invite all like-minded countries to consider joining us in our actions," they said in a communique.
The United States also issued a new round of sanctions that prohibit imported Russian gold as well as target Russia's state-owned defence conglomerate, Rostec, and multiple banks.
With summit action now shifting to NATO, White House National Security Adviser Jake Sullivan said a new strategic concept would "describe in stark terms the threat that Russia poses and the way in which it has shattered peace in Europe".
That marks a departure from post-Soviet NATO policy which cast Russia as a potential partner.
Russia denies intentionally targeting civilians in its "special military operation" that has destroyed Ukrainian cities, killed thousands of people and driven millions from their homes.
The attack on Kremenchuk came after days of increasing Russian missile strikes a long distance from the frontline, including the first attacks on the capital Kyiv for weeks.
The United Nations Security Council, where Russia wields a veto, will meet later on Tuesday at Ukraine's request following the Kremenchuk attack.
