The United States vowed to reinforce Europe's defences in the wake of Russia's invasion of Ukraine, as NATO declared Moscow the West's greatest threat -- prompting Vladimir Putin to lash out at the alliance's "imperial ambitions".

Meeting in Madrid Wednesday, NATO leaders said Russia "is the most significant and direct threat to allies' security and to peace and stability in the Euro-Atlantic area".

This came as NATO officially invited Sweden and Finland to join the alliance, and US President Joe Biden announced new deployments of US troops, ships and planes to Europe.

A Russian missile strike on a shopping mall in Kremenchuk sparked Western outrage
A Russian missile strike on a shopping mall in Kremenchuk sparked Western outrage AFP / GENYA SAVILOV

Biden said that the US move was exactly what Russian President Putin "didn't want" -- and Moscow, facing fierce resistance from Ukrainian forces equipped with Western arms, reacted with predictable fury.

Putin accused the alliance of seeking to assert its "supremacy", telling journalists in the Turkmenistan capital of Ashgabat that Ukraine and its people are "a means" for NATO to "defend their own interests."

"The NATO countries' leaders wish to... assert their supremacy, their imperial ambitions," the Russian president added.

'The G7 stands united in its support for Ukraine,' Germany's Olaf Scholz said
'The G7 stands united in its support for Ukraine,' Germany's Olaf Scholz said AFP / Ronny HARTMANN

NATO leaders have funnelled billions of dollars of arms to Ukraine and faced a renewed appeal from President Volodymyr Zelensky for more long-range artillery.

"Ukraine can count on us for as long as it takes," NATO chief Jens Stoltenberg said at the summit, which ends Thursday, announcing a new strategic overview that focuses on the Moscow threat.

The document, updated for the first time since 2010, warned that the alliance "cannot discount the possibility" of an attack on its members.

NATO allies are expected to pledge more arms supplies to Ukrainian fighters
NATO allies are expected to pledge more arms supplies to Ukrainian fighters AFP / Bagus SARAGIH

Ukraine's Foreign Minister Dmytro Kuleba welcomed NATO's "clear-eyed stance on Russia".

NATO allies are expected to pledge more arms supplies to Ukrainian fighters
NATO allies are expected to pledge more arms supplies to Ukrainian fighters AFP / Bagus SARAGIH

Russian missiles continued to rain down across Ukraine. Zelensky said that a missile strike on the southern city of Mykolaiv destroyed a five-storey building, killing at least five people.

The city of Lysychansk in the eastern Donbas region -- the current focus of Russia's offensive -- was also facing sustained bombardment.

"The Russians are throwing almost all of their resources at capturing Lysychansk," Sergiy Gaiday, regional governor of Lugansk, which includes the city, said on Telegram.

In Kremenchuk, the town where a Russian missile on Monday destroyed a shopping centre and -- according to local officials -- killed at least 18 civilians, clearing operations continued
In Kremenchuk, the town where a Russian missile on Monday destroyed a shopping centre and -- according to local officials -- killed at least 18 civilians, clearing operations continued AFP / Genya SAVILOV

"It's hard to find a safe spot in the city."

The Lysychansk refinery also has come under attack, he added.

On Thursday, a Moscow-appointed official said a ship carrying grain had sailed from the occupied Ukrainian port of Berdyansk, marking the first grain shipment since the start of hostilities.

Charts showing NATO's military spending and personnel
Charts showing NATO's military spending and personnel AFP / Gal ROMA

"After numerous months of delay, the first merchant ship has left the Berdyansk commercial port, 7,000 tonnes of grain are heading toward friendly countries," Evgeny Balitsky, the head of the pro-Russia administration, said on Telegram.

Ukraine has accused Russia and its allies of stealing its grain, contributing to a global food shortage caused by a blockade of exports in Ukrainian ports.

In Kremenchuk, the town where a Russian missile on Monday destroyed a shopping centre and killed at least 18 civilians, clearing operations continued.

A giant crane was working near the site of impact, and in the rubble-strewn parking area, shopping trolleys piled with clothes and household goods lay abandoned.

At a hospital in the city, some of the wounded recalled the moment of the attack.

"We didn't hear the sound of the missile -- a sudden clap, flash, and we got blown away," said Petr Ozhereliev, an employee at the mall.

"I guess I lost consciousness, because when I woke up I was crawling out of the rubble."

Western leaders have dubbed the Kremenchuk strike a war crime. Russia says it hit a depot storing Western arms, and Putin on Wednesday denied Moscow's forces were responsible for the attack.

Meanwhile, Ukrainian officials said that 144 of their soldiers, most of them former defenders of the Azovstal steelworks in the southern port city of Mariupol, had been freed in a prisoner swap with Moscow.

Moscow's invasion triggered massive economic sanctions and a wave of support for Zelensky's government, including deliveries of advanced weapons, as well as the reinforcement of Europe's defences.

Washington has announced that it will shift the headquarters of its 5th Army Corps to Poland.

An army brigade will rotate in and out of Romania, two squadrons of F-35 fighters will deploy to Britain, US air defence systems will be sent to Germany and Italy, and the fleet of US Navy destroyers in Spain will grow from four to six.

Britain also pledged another $1.2 billion in military aid for Ukraine on Wednesday, including air defence systems and drones.

"Until now, we were speaking about an alleged war crime. Now we can clearly say it was one, committed by the Russian armed forces," Oksana Pokalchuk, head of Amnesty's Ukraine branch, told AFP.

Nevertheless, the group also found the death toll may have been smaller than initially believed. Amnesty believes at least a dozen people died in the attack although it is likely many additional fatalities remain unreported.

Mariupol city authorities had provided an initial estimate of around 300 deaths.