Scores of civilians were feared killed or wounded in a Russian missile strike Monday on a crowded shopping mall in Ukraine's central city of Kremenchuk, Ukrainian officials said.

Ukrainian President Volodymyr Zelenskyy said in a Telegram post that the number of victims was "unimaginable," citing reports that more than 1,000 civilians were inside at the time of the attack.

Images from the scene showed giant plumes of black smoke from the shopping centre engulfed in flames, as emergency crews rushed in and onlookers watched in distress.

At least 13 people were dead and more than 40 wounded, according to the regional governor, Dmytro Lunin, who said rescuers were continuing to comb the smouldering debris for more victims.

The strike unfolded as Western leaders pledged continued support for Ukraine, and the world's major economies got ready to pursue new sanctions on Russia, including a price cap on oil and higher tariffs on goods.

Meanwhile, the US appeared ready to respond to Zelenskyy's call for more air defence systems, and NATO planned to increase the size of its rapid-reaction forces nearly eightfold – to 300,000 troops.

Zelenskyy said the mall 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."

The Ukrainian military said the shopping centre was hit by missiles fired by Russian Tu-22M3 long-range bombers from the skies over Russia's western Kursk region.

The secretary of Ukraine's National Security and Defense Council, Oleksiy Danilov, said one missile hit the shopping centre and another struck a sports arena in Kremenchuk.

The Russian strike carried echoes of attacks earlier in the war that caused large numbers of civilian casualties – such as one in March on a Mariupol theatre where many civilians had holed up, killing an estimated 600, and another in April on a train station in eastern Kramatorsk that left at least 59 people dead.

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

Mayor Vitaliy Maletskiy wrote on Facebook that the attack "hit a very crowded area, which is 100 percent certain not to have any links to the armed forces."

The United Nations called the attack on the shopping centre "deplorable," stressing that civilian infrastructure "should never ever be targeted.

The attack happened as Russia was mounting an all-out assault on the last Ukrainian stronghold in eastern Ukraine's Luhansk province, "pouring fire" on the city of Lysychansk from the ground and air, according to the local governor.

At least eight people were killed and more than 20 wounded in Lysychansk when Russian rockets hit an area where a crowd of people gathered to get water from a tank, Luhansk Governor Serhiy Haidai said.

Russian forces appeared to step up an offensive centring on trying to wrest the eastern Donbas region from Ukraine after forcing government troops out of the neighbouring city of Sievierodonetsk in recent days.

To the west of Lysychansk Monday, the mayor of the city of Sloviansk – potentially the next major battleground – said Russian forces fired cluster munitions on the city after dawn, including one that hit a residential neighbourhood.

Authorities said the number of dead and wounded had yet to be confirmed. The Associated Press saw one fatality: A man's body lay hunched over a car door frame, his blood pooling onto the ground from chest and head wounds.

The blast blew out most windows in the surrounding apartment blocks and the cars parked below, littering the ground with broken glass.

"Everything is now destroyed. We are the only people left living in this part of the building. There is no power," said local resident Valentina Vitkovska, in tears as she spoke about the blast. "I can't even call to tell others what had happened to us."

Overall, Zelenskyy's office said at least six civilians were killed and 31 others wounded as part of intense Russian shelling against various Ukrainian cities over the past 24 hours – including Kyiv and major cities in the country's south and east, but not counting the attack in Kremenchuk and the shelling of the eastern city of Kharkiv where at least five people were killed and another 15 were wounded.

It said Russian forces fired rockets that killed two people and wounded five overnight in and near Kharkiv, Ukraine's second-largest city, and continued to target the key southern port of Odesa. A missile attack destroyed residential buildings and wounded six people, including a child.

In Lysychansk, at least five high-rise buildings in the city and the last road bridge were damaged over the past day, the regional governor said.

A crucial highway linking the city to the government-held territory to the south was rendered impassable by shelling.

The city had a prewar population of around 100,000, around one-tenth of whom remain.

Analysts say that Lysychansk's location high on the banks of the Siverskyy Donets River gives a major advantage to the city's Ukrainian defenders.

"It's a very hard nut to crack. The Russians could spend many months and much effort storming Lysychansk," said military analyst Oleh Zhdanov.

In other developments, in Germany's Bavarian Alps, leaders of the Group of Seven countries unveiled plans to seek new sanctions and pledged to continue supporting Ukraine "for as long as it takes."

In a joint statement Monday after they held a session by video link with Zelenskyy, the leaders underlined their "unwavering commitment to supporting the government and people of Ukraine in their courageous defence of their country's sovereignty and territorial integrity."

Elsewhere, Washington was expected to announce the purchase of an advanced surface-to-air missile system for Ukraine.

In Brussels, NATO Secretary-General Jens Stoltenberg announced the plans to greatly expand the alliance's rapid-reaction forces as part of its response to an "era of strategic competition." The NATO response force now has about 40,000 soldiers.

NATO will agree to deliver further military support to Ukraine – including secure communication and anti-drone systems – when its leaders convene in Spain for a summit, Stoltenberg said.

Britain's defence ministry said Russia is likely to rely increasingly on reserve forces in the coming weeks of the war.

Analysts have said a call-up of reservists by Russia could vastly alter the balance in the war but could also come with political consequences for President Vladimir Putin's government.

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