World leaders have denounced Russia’s deadly strike on a shopping centre in the Ukrainian city of Kremenchuk as “abominable” and a war crime.

The search for survivors continues after the missile strike on a mall that had hundreds of people in it at the time, according to Ukraine’s president, Volodoymyr Zelenskiy.

The Ukrainian defence ministry said the attack, which is likely to become responsible for one of the war’s highest civilian death tolls in a single strike, was deliberately timed to coincide with the mall’s busiest hours and cause the maximum number of casualties.

Twenty people are known to have been killed, according to Ukraine’s emergency services, and Serhiy Kruk, the head of Ukraine’s state emergency service, said 59 had been injured. Ukrainian prosecutors said they also found the remains of a further six people.

Ukraine’s interior minister, Denys Monastyrskiy, said in a briefing that 21 people were still missing, although he added that some of the missing people could be among the bodies that had been found.

Authorities estimate there were between 200 and 1,000 people inside the mall. Many managed to flee to a nearby bomb shelter when they heard an air raid siren.

“I left the building two minutes before the explosion,” said Yevhenia Semyonova, 38, a shop assistant at a sportswear store in the mall. “My colleagues who are working in bigger stores, like the supermarket, for example, had to wait for the customers to get out before they could leave. We were lucky because there were no customers in our story during the alarm.”

In a joint statement, the leaders of the G7 condemned the “abominable attack” and noted that strikes aimed at civilians were a war crime, and they pledged “unwavering support” for Ukraine.

“We stand united with Ukraine in mourning the innocent victims of this brutal attack. Indiscriminate attacks on innocent civilians constitute a war crime. [Vladimir] Putin and those responsible will be held to account,” the statement said. “We will not rest until Russia ends its cruel and senseless war on Ukraine.”

Separately, the French president, Emmanuel Macron, called the attack an “abomination”. He said: “We share the pain of the victims’ families, and the anger in the face of such an atrocity. The Russian people have to see the truth.”

A UN spokesperson, Stéphane Dujarric, said: “It is deplorable, to say the least. Any sort of civilian infrastructure, which includes obviously shopping malls, and civilians should never ever be targeted.”

Russia’s ministry of defence said on Tuesday morning that the fire was caused by “the detonation of stored ammunition for western weapons” located next to the mall, which Moscow claimed was not operating.

The ministry said it conducted a “high-precision” strike on a hangar in Kremenchuk containing weapons supplied by US and European countries. No evidence was offered to back up the claims.

Dmitry Peskov, the Kremlin’s spokesperson, told reporters on Tuesday that the war would end only when Ukraine capitulated to Moscow’s demands. “The Ukrainian side can end everything by the end of today. They just need to order Ukrainian troops to lay down their weapons and fulfil Russia’s conditions. And it will all be over,” he said.

“An order for the nationalist units to lay down their arms is necessary.”

Ukraine’s air force command said Russia struck the mall, located near a railway station, with two X-22 cruise missiles that were fired by an unspecified number of Tu-22M long-range bombers.

Images from the scene showed plumes of black smoke and flames, and emergency crews rushing in to search for victims. It took 300 emergency workers more than four hours to extinguish the flames.

Mykola Lukash, from the Kremenchuk district prosecutor’s office, said cranes would be brought in on Tuesday to help lift the collapsed roof of the shopping centre. “We haven’t found any children’s bodies. A lot of bodies are burnt. We need to carry out DNA tests,” Lukash said.

On Monday night, emergency workers and soldiers combed through blackened debris and twisted metal. “We pulled out several bodies, but there are definitely more trapped under the rubble,” said Oleksii, 46, a firefighter. “This is normally a very crowded place.”

Speaking at 2am local time, Kruk, the emergency services chief, said work was continuing at the site. He added: “The main tasks currently performed by rescuers are to carry out rescue operations, dismantle debris and eliminate fires. So far, 16 people have been killed and 59 injured, 25 of whom have been hospitalised.”

Zelenskiy said on Telegram that the number of victims was “unimaginable”. He wrote: “The occupiers fired missiles at the shopping centre, where there were more than a thousand civilians. The mall is on fire, rescuers are extinguishing the fire, the number of victims is unimaginable. Russia continues to take out its impotence on ordinary citizens. It is useless to hope for decency and humanity from Russia.”

In his nightly video address, Zelenskiy also called Russia “the largest terrorist organisation in the world”.

Elsewhere in the country, Russia continued its efforts to capture Lysychansk, the last Ukrainian stronghold in the eastern Luhansk region, after Ukrainian forces were forced to retreat from twin city Sievierodonetsk last week.

Serhiy Haidai, the governor of Luhansk province, said on Telegram that Russian forces were storming Lysychansk from the south and south-west. “Fighting is currently under way in the area … the Russians are obliterating Luhansk residents,” he said.

A Russian missile strike killed eight and wounded 21 others in Lysychansk on Monday, Haidai said.

If the city of Lysychansk falls, the entire region of Luhansk, which along with Donetsk makes up the eastern Donbas region, would come under Russian control, making it the first Ukrainian region to be occupied by Russia since the country annexed Crimea in 2014.

Regional heads in the second largest city of Kharkiv, the central city of Dnipro and the southern city of Mykolayiv also reported renewed Russian shelling on Tuesday afternoon, as the conflict shows no sign of abating.