Ukrainian forces braced Sunday to defend their final bastion in the devastated port city of Mariupol, desperate to deny Russia a symbolic win on the eve of Moscow's Victory Day celebrations.

Shelling and missile strikes have intensified in the build up to the World War II anniversary, and rescuers are searching for 60 Ukrainian civilians feared killed in the bombing of a village school.

President Volodymyr Zelenskyy marked the anniversary of the end of the World War by comparing Ukraine's battle for national survival to the region's war of resistance against its former Nazi occupiers.

"Decades after World War II, darkness has returned to Ukraine, and it has become black and white again," Zelenskyy said, in a monochrome social media video shot against the backdrop of a bombed-out apartment block.

"Evil has returned, in a different uniform, under different slogans, but for the same purpose," he warned, trying to turn Russian President Vladimir Putin's "anti-Nazi" rhetoric back on itself.

Russia, meanwhile, was gearing up for a Victory Day parade designed to associate the invasion of its neighbor with the national pride felt over the Soviet Union's defeat of Germany.

"Today, our soldiers, as their ancestors, are fighting side by side to liberate their native land from the Nazi filth with the confidence that, as in 1945, victory will be ours," Putin said.

Zelenskyy was also to meet G-7 leaders via videoconference to discuss the crisis, and European diplomats will meet again next week to hammer out the details of their latest sanctions package against Moscow.

On the ground, the key battles were being fought in Ukraine's east.

Tunnel network

Civilians have now been evacuated from Mariupol's Azovstal steelworks, leaving a small force of defenders holed up in its sprawling network of underground tunnels and bunkers.

The complex – the final pocket of Ukrainian resistance in the port city – has taken on a symbolic value.

"We, all of the military personnel in the garnison of Mariupol, we have witnessed the war crimes performed by Russia, by the Russian army. We are witnesses," said Ilya Samoilenko, an intelligence officer with the far-right Azov Regiment, which is defending the steelworks.

"Surrender is not an option because Russia is not interested in our lives," he said.

Taking full control of Mariupol would also allow Moscow to create a land bridge between the Crimean Peninsula, which it annexed in 2014, and regions run by pro-Russian separatists in the east.

In one of those regions, Luhansk, Ukrainian forces are now mounting a last ditch defense of the city of Severodonetsk, formerly an industrial city of 100,000 people, now Russia's next target.

In the same region, Governor Sergiy Gaiday said 60 civilians were feared dead after a school in the village of Bilogorivka was hit in an airstrike.

"The bombs fell on the school and unfortunately it was completely destroyed. There were a total of 90 people, 27 were saved," he said on Telegram.

"Sixty people who were in the school are very probably dead."

Rescuers could not work overnight because of a threat of new strikes, but resumed their work on Sunday.

'Filtration camps'

Rescuers were also looking for survivors in the neighboring village of Shepilivka after a strike hit a house where 11 people were sheltering in the basement, Gaiday said.

Civilians who escape Mariupol describe passing through Russian "filtration" sites where several evacuees told Agence France-Presse (AFP) they were questioned, strip-searched, fingerprinted and had their phones and documents checked.

"They asked us if we wanted to go to Russia ... or stay and rebuild the city of Mariupol," said Azovstal evacuee Natalia, who spoke on condition that her full name not be published.

"But how can I rebuild it? How can I return there if the city of Mariupol doesn't exist anymore?"

Russia's campaign has run into tough resistance – and galvanized Kyiv's Western allies to impose potentially crippling sanctions on the Russian economy and Putin's inner circle.

International efforts to pressure the Russian leader continue, with G-7 leaders, including U.S. President Joe Biden, to talk with Zelenskyy on Sunday to discuss Western support for Kyiv.

German Chancellor Olaf Scholz will host the video call and Zelenskyy will "take part and report on the current situation," spokesperson Christiane Hoffmann said.

Further sanctions or at least a tightening of the huge array of economic punishments already inflicted on Russia are expected to be discussed.