As Moscow celebrated its national Victory Day on Monday, Russian forces pushed forward in their assault on Ukraine, seeking to capture the devastated port city of Mariupol.

Putin evoked the memory of Soviet heroism in World War II to urge his army towards victory in Ukraine but acknowledged the cost in Russian lives as he pledged to help the families of fallen soldiers.

Addressing massed ranks of service personnel on Red Square on the 77th anniversary of victory over Nazi Germany, Putin condemned what he called external threats to weaken and split Russia.

Putin said Russia's "special military operation" was a purely defensive and unavoidable measure against plans for a NATO-backed invasion of lands he said were historically Russia's, including Crimea. "Russia preventively rebuffed the aggressor," he said, offering no evidence for what he called open preparations to attack Crimea and Ukraine's Donbass region.

He directly addressed soldiers fighting in the Donbass region of eastern Ukraine, which Russia has pledged to "liberate" from Kyiv's control.

"You are fighting for the Motherland, for its future, so that no one forgets the lessons of World War II. So that there is no place in the world for executioners, punishers and Nazis," he said.

His speech included a minute of silence. "The death of each one of our soldiers and officers is our shared grief and an irreparable loss for their friends and relatives," said Putin, promising that the state would look after their children and families.

But his 11-minute speech, on day 75 of the invasion, was largely notable for what he did not say.

He did not mention Ukraine by name, gave no assessment of progress in the war and offered no indication of how long it might continue. There was no mention of the bloody battle for Mariupol, where Ukrainian defenders holed up in the ruins of the Azovstal steel works are still defying Russia's assault.

Putin has repeatedly likened the war – 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's President Volodymyr Zelenskyy has said it is Russia that is staging a "bloody reenactment of Nazism" in Ukraine.

Zelenskyy declared in his own Victory Day address that his country would eventually defeat the Russians. "Very soon there will be two Victory Days in Ukraine," he said in a video released to mark the holiday.

"We have never fought against anyone. We always fight for ourselves ... We are fighting for freedom for our children, and therefore we will win."

Determined to show a success in a war now in its 11th week, Russian troops have targeted a sprawling seaside steel mill where an estimated 2,000 Ukrainian fighters were making what appeared to be their last stand to save Mariupol from falling.

The mill is the only part of the city not overtaken by the invaders, and its defeat would deprive Ukraine of a vital port and allow Russia to establish a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014.

Zelenskyy confirmed the deaths of dozens of people in the Russian bombing of a school in 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," Zelenskyy said in his nightly video address.

About 90 people had taken refuge at the school, the governor of Ukraine's eastern Luhansk region Serhiy Gaidai had said. There was no response from Moscow, which says it does not target civilians.

"They have nothing to celebrate," Linda Thomas-Greenfield, U.S. ambassador to the United Nations, said of the Russians, speaking on CNN. "They have not succeeded in defeating the Ukrainians. They have not succeeded in dividing the world or dividing NATO. And they have only succeeded in isolating themselves internationally and becoming a pariah state around the globe."

Though fighting continues on multiple fronts, Russia is closest to victory in Mariupol. Ukrainian fighters in the steel mill have rejected deadlines set by the Russians for laying down their arms even as attacks continued by warplanes, artillery and tanks.

On Ukraine’s coast, explosions echoed again across the major Black Sea port of Odessa. The Ukrainian military said Moscow was focusing its main efforts on destroying airfield infrastructure in eastern and southern Ukraine.

In a sign of the dogged resistance that has sustained the fighting into its 11th week, Ukraine’s military struck Russian positions on a Black Sea island that was captured in the war’s first days. A satellite image by Planet Labs showed smoke rising from two sites on the island.

But Moscow’s forces showed no sign of backing down in the south. Satellite photos show Russia has put armored vehicles and missile systems at a small base in the Crimean Peninsula.

The most intense combat in recent days has taken place in eastern Ukraine. A Ukrainian counteroffensive in the northeast near Kharkiv, the country’s second-largest city, is making "significant progress," according to the Institute for the Study of War, a Washington think tank.

However, the Ukrainian army withdrew from the embattled eastern city of Popasna, regional authorities said.

Rodion Miroshnik, a representative of the pro-Kremlin, separatist Luhansk People’s Republic, said its forces and Russian troops had captured most of Popasna after two months of fierce fighting.

The Kharkiv regional administration said three people were killed in the shelling of the town of Bogodukhiv, about 50 kilometers (30 miles) from Kharkiv.

South of Kharkiv, in Dnipropetrovsk province, the governor said a 12-year-old boy was killed by a cluster munition that he found after a Russian attack. An international treaty bans the use of such explosives, but neither Russia nor Ukraine has signed the agreement.

"This war is treacherous," the governor, Valentyn Reznichenko, wrote on social media. "It is near, even when it is invisible."

The Group of Seven industrial democracies pledged to ban or phase out imports of Russian oil. The G-7 consists of the U.S., Canada, Britain, Germany, France, Italy and Japan.

The United States also announced new sanctions against Russia, cutting off Western advertising from Russia’s three biggest TV stations, banning U.S. accounting and consulting firms from providing services, and cutting off Russia’s industrial sector from wood products, industrial engines, boilers and bulldozers.

U.S. first lady Jill Biden met with her Ukrainian counterpart. Canadian Prime Minister Justin Trudeau raised his country’s flag at its embassy in Kyiv. And U2′s Bono, alongside bandmate The Edge, performed in a Kyiv subway station that had been used as a bomb shelter, singing the 1960s song "Stand by Me."

The acting U.S. ambassador to Ukraine, Kristina Kvien, posted a picture of herself at the American Embassy, and described plans for the eventual U.S. return to the Ukrainian capital after Moscow’s forces abandoned their effort to storm Kyiv weeks ago and began focusing on the capture of Donbass.