Firefighters and soldiers searched on Tuesday for survivors in the rubble of a Ukrainian shopping mall, where authorities said 36 people were still missing after a Russian missile strike that had killed at least 18.

Relatives of the missing were lined up on Tuesday at a hotel across the street from the wreckage of the shopping centre, where rescue workers had set up a base.

Exhausted-looking firefighters sat on a kerb after a night battling the blaze and searching for survivors, mostly in vain. Oleksandr, wetting his face from a water bottle on a bench, said his team had worked all night picking through the rubble.

"We pulled out five bodies. We didn't find anybody alive," he said.

Ukrainian President Volodymyr Zelenskiy accused Russia of deliberately targetting civilians in the attack in the city of Kremenchuk in central Ukraine, far from the frontline. He called it "one of the most defiant terrorist attacks in European history".

Russia said the incident was caused by a strike on a legitimate military target. Its defence ministry said it had fired missiles at an arms depot for Western weapons, where an explosion of ammunition caused the blaze at the nearby mall.

Moscow said the mall was disused and empty at the time of the strike, a claim plainly contradicted by 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 nearby public hospital where she was being treated.

"It was hell," said her husband, Mykola, 45, blood seeping through a bandage around his head.

Leaders of the Group of Seven (G7) major democracies, at a summit in Germany, said the attack was "abominable".

"Russian President (Vladimir) Putin and those responsible will be held to account," they said in a joint statement.

Ukraine's Prosecutor General Iryna Venediktova told Reuters another missile had also struck a nearby factory, which was closed and not a military target.

"It's a question about crimes against humanity," she said. "I think it's like systematical shelling of civilian infrastructure - with what aim? To scare people, to kill people to make terror in our cities and villages."

BATTLE FOR LYSYCHANSK

Russia denies intentionally targetting civilians in its "special military operation" which has destroyed cities, killed thousands of people and driven millions from their homes.

The attack on Kremenchuk comes after days of increasing Russian missile strikes far from the frontline, including the first attacks on the capital Kyiv for weeks.

Moscow has also stepped up shelling of Kharkiv, Ukraine's second largest city, where Russian troops were pushed back in a counter-offensive in May. The Kharkiv governor said five people were killed and 22 wounded in shelling on Monday that hit targets including apartment buildings and a school.

The U.N. Security Council, where Moscow wields a veto, will meet on Tuesday at Ukraine's request following the Kremenchuk attack. U.N. spokesperson Stephane Dujarric said the missile strike was deplorable.

On the battlefields of the Donbas region, Ukraine endured another difficult day following the loss last week of the now-ruined city of Sievierodonetsk.

Russian forces are trying to storm Lysychansk, across the Siverskyi Donets River from Sievierodonetsk, which would complete their capture of Luhansk province, one of two eastern regions Moscow aims to conquer on behalf of separatist proxies.

Eight residents including a child were killed and 21 wounded by shelling when they gathered to get drinking water in Lysychansk on Monday, Luhansk Governor Serhiy Gaidai said.

Ukrainian forces still controlled the city but its loss was possible as Russia poured resources into the fight, he added.

"They really want this and a lot of reserves are being thrown just for this...We do not need to lose an army for the sake of one city," he told Reuters.

Rodion Miroshnik, ambassador to Moscow of the Luhansk separatists, said Russian and separatist troops were advancing westward into Lysychansk and street battles had erupted around the city stadium. Fighting was going on in several surrounding villages, and the pro-Russian fighters had entered an oil refinery where Ukrainian troops were concentrated, Miroshnik said on Telegram.

The advances in Luhansk have given Moscow its biggest victories in weeks, playing to Russia's strength in overwhelming artillery firepower that can demolish cities in its path.

But Ukraine still hopes to turn the tide in coming weeks with a counterattack after Moscow exhausts its invading forces in costly battles that yield slow gains. Kyiv has begun to receive more advanced Western weapons, including battlefield rockets that can hit targets far behind the front.

The Ukraine war will be the main topic when leaders of NATO countries hold an annual summit this week in Spain.

Britain's new military chief, General Patrick Sanders, said whatever the outcome of the war, Russia was likely to emerge as a greater threat to the West than before.

"While Russia's conventional capability will be much reduced for a time at least, Putin's declared intent recently to restore the lands of historic Russia makes any respite temporary and the threat will become even more acute," he said in a speech.