Ukrainians were urged to heed air raid sirens and stay in bomb shelters on Monday by officials concerned that Russian attacks would intensify as Moscow marked the anniversary of the Soviet Union's victory over Nazi Germany in World War Two.

President Vladimir Putin presided over a display of Russia's vast firepower in Moscow, more than 10 weeks into an invasion of its neighbour it terms a "special military operation". He told troops they were fighting for Russia's security now.

Russia's intervention in Ukraine was needed because the West was "preparing for the invasion of our land, including Crimea", Putin said, referring to the Black Sea peninsula Russia annexed from Ukraine in 2014 and giving no evidence for his assertion.

The governor of Ukraine's eastern Luhansk region, Serhiy Gaidai, said: "Today we do not know what to expect from the enemy, what terrible thing they might do, so please go out onto the street as little as possible, stay in the shelters."

Ukrainian President Volodymyr Zelenskiy earlier confirmed the deaths of dozens of people in the Russian bombing of a school in Bilohorivka, eastern Ukraine on Saturday.

"As a result of a Russian strike on Bilohorivka in the Luhansk region, about 60 people were killed, civilians, who simply hid at the school, sheltering from shelling," Zelenskiy said in his nightly video address.

About 90 people had taken refuge at the school, Gaidai had said. There was no response from Moscow. 

In the southern port of Mariupol, which has endured the most destruction of the 10-week war, the deputy commander of the Azov regiment holed up in the Azovstal steel plant pleaded with the international community to help evacuate wounded soldiers.

"We will continue to fight as long as we are alive to repel the Russian occupiers," Captain Sviatoslav Palamar told an online news conference.

Zelenskiy said his country would win against Russia and would not cede any territory.

"There is no invader who can rule over our free people. Sooner or later we will win," he said in a written address to mark the World War Two victory anniversary.

Leaders of the Group of Seven (G7) industrial nations vowed on Sunday to deepen Russia's economic isolation and "elevate" a campaign against Kremlin-linked elites. 

The G7 said it was committed to phasing out or banning Russian oil and denounced Putin's invasion of Ukraine.

"His actions bring shame on Russia and the historic sacrifices of its people," the group said, referring to Soviet Russia's role in defeating Nazi Germany 77 years ago.

Putin has repeatedly likened the war in Ukraine - which he casts as a battle against dangerous "Nazi"-inspired nationalists in Ukraine - to the challenge the Soviet Union faced when Adolf Hitler invaded in 1941.

Ukraine and its allies reject the accusation of Nazism and the assertion that Russia is fighting for survival against an aggressive West, saying Putin unleashed an unprovoked war in an attempt to rebuild the Soviet Union.

Ahead of the military parade, Russia's deputy prime minister Yuri Borisov said the country was developing new-generation hypersonic missiles and had enough high-precision missiles and ammunition to fulfil all the tasks assigned to its armed forces.

Moscow has come under increasingly punishing sanctions since its invasion on Feb. 24, with trade heavily impacted and assets seized.

The European Union's foreign policy chief said the bloc should consider using frozen Russian foreign exchange reserves to help pay for the cost of rebuilding Ukraine after the war. Josep Borrell was speaking to the Financial Times. read more

In the Ukrainian-controlled city of Zaporizhzhia, about 230 km (140 miles) northwest of Mariupol, dozens of people who had fled the city and nearby occupied areas waited to register in a car park set up for evacuees.

"There's lots of people still in Mariupol who want to leave but can't," said history teacher Viktoria Andreyeva, 46, who said she had only just reached the city after leaving her bombed home in Mariupol with her family in mid-April.

"The air feels different here, free," she said in a tent where volunteers offered food, basic supplies and toys to the evacuees, many travelling with small children.

Separatists said a total of 408 people were evacuated from Mariupol over the past 24 hours, including 65 children.

Mariupol is key to Moscow's efforts to link the Crimean Peninsula, seized by Russia in 2014, and parts of the eastern regions of Luhansk and Donetsk that have been controlled by Russia-backed separatists since then.

In Luhansk and Donetsk regions, half a dozen Russian attacks were repulsed, with tanks and armoured combat vehicles destroyed, governor Gaidai said on Monday.

Viktor Andrusiv, an adviser to the interior minister, said Ukraine was awaiting the delivery of more sophisticated weapons and expecting further attacks from Russia on Monday.

"We are preparing for rocket attacks today - please, take air alerts very responsibly today."

A number of Western officials, including U.S. First Lady Jill Biden, Canadian Prime Minister Justin Trudeau, a German parliament head and the Norwegian foreign minister arrived in Ukraine on Sunday in a show of support. read more

Irish rock group U2's frontman Bono and his bandmate The Edge performed a 40-minute concert in a metro station in Kyiv on Sunday and praised Ukrainians fighting for their freedom.

"They can take your lives, but they can never take your pride," Bono said.

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