Ukrainian authorities said the Russian military bombed an art school where about 400 people had taken refuge in the port city of Mariupol, where President Volodymyr Zelenskyy said an unrelenting siege by Russian troops would go down in history for what he said were war crimes.

Local authorities said the school’s building was destroyed and people could remain under the rubble. There was no immediate word on casualties. Russian forces on Wednesday also bombed a theater in Mariupol where civilians were sheltering, authorities have said.

“To do this to a peaceful city, what the occupiers did, is a terror that will be remembered for centuries to come,” Zelenskyy said in his nightly video address to the nation.

Mariupol, a strategic port on the Azov Sea, has been under bombardment for at least three weeks and become a symbol of the horror of Russia’s war in Ukraine. Local authorities have said the siege has cut off food, water and energy supplies, and killed at least 2,300 people, some of whom had to be buried in mass graves.

Russian forces have surrounded the battered city and pushed deeper into it in recent days. Heavy fighting shut down a major steel plant and local authorities pleaded for more Western help Saturday.

“Children, elderly people are dying. The city is destroyed and it is wiped off the face of the earth,” Mariupol police officer Michail Vershnin said from a rubble-strewn street in a video addressed to Western leaders that was authenticated by The Associated Press.

The fall of Mariupol, the scene of some of the war’s worst suffering, would mark a major battlefield advance for the Russians, whose advance is largely stalled outside other major cities more than three weeks into the biggest land invasion in Europe since World War II.

In the capital, Kyiv, at least 20 babies carried by Ukrainian surrogate mothers are stuck in a makeshift bomb shelter, waiting for parents to travel into the war zone to pick them up. Some just days old, the babies are being cared for by nurses who cannot leave the shelter because of constant shelling by Russian troops who are trying to encircle the city.

Details also began to emerge about a rocket attack that killed as many as 40 marines in the Black Sea port city of Mykolaiv on Friday, according to a Ukrainian military official who spoke to The New York Times. It wasn’t clear how many marines were inside at the time, and rescuers continued searching the rubble of the barracks.

A senior Ukrainian military official, who spoke to the Times on condition of anonymity to reveal sensitive information, estimated that as many as 40 marines were killed, which would make it one of the deadliest known attacks on Ukrainian forces during the war.

Meanwhile the Russian military reported Sunday that it had carried out a new series of strikes on Ukrainian military facilities with long-range hypersonic and cruise missiles.

The Russian Defense Ministry’s spokesman, Maj. Gen. Igor Konashenkov, said a Kinzhal hypersonic missile hit a Ukrainian fuel depot in Kostiantynivka, a city near Mykolaiv. The Russian military said Saturday that it used a Kinzhal for the first time in combat to destroy an ammunition depot in Diliatyn in the Carpathian Mountains in western Ukraine.

Russia has said the Kinzhal, carried by MiG-31 fighter jets, has a range of up to 2,000 kilometers (about 1,250 miles) and flies at 10 times the speed of sound. Pentagon press secretary John Kirby said Saturday that the U.S. couldn’t confirm the use of a hypersonic missile in Ukraine.

Konashenkov said Kalibr cruise missiles launched by Russian warships from the Caspian Sea were also involved in the strike on the fuel depot in Kostiantynivka and were used to destroy an armor repair plant in Nizhyn in the Chernihiv region in northern Ukraine.

Despite the siege in Mariupol and the geographic scope of Russia’s assault, many remained struck by Ukraine’s ability to hold back its much bigger, better-armed foe. The United Kingdom’s Defense Ministry said Ukraine’s airspace continued to be effectively defended.

“Gaining control of the air was one of Russia’s principal objectives for the opening days of the conflict and their continued failure to do so has significantly blunted their operational progress,” the ministry said on Twitter.

Russia is now relying on stand-off weapons launched from the relative safety of Russian airspace to strike targets within Ukraine, the British ministry said.

Estimates of Russian deaths vary widely, but even conservative figures are in the low thousands. Russia had 64 deaths in five days of fighting during its 2008 war with Georgia. It lost about 15,000 in Afghanistan over 10 years, and more than 11,000 in years of fighting in Chechnya.

Russia’s number of dead and wounded in Ukraine is nearing the 10% benchmark of diminished combat effectiveness, said Dmitry Gorenburg, a researcher on Russia’s security at the Virginia-based CNA think tank. The reported battlefield deaths of four Russian generals — out of an estimated 20 in the fight — signal impaired command, Gorenburg said.

Russia would need 800,000 troops — almost equal to its entire active-duty military — to control Ukraine long-term in the face of armed opposition, said Michael Clarke, former head of the British-based Royal United Services Institute, a defense think tank.

“Unless the Russians intend to be completely genocidal — they could flatten all the major cities, and Ukrainians will rise up against Russian occupation — there will be just constant guerrilla war,” said Clarke.

U.N. bodies have confirmed more than 847 civilian deaths since the war began, though they concede the actual toll is likely much higher. The U.N. says more than 3.3 million people have fled Ukraine as refugees.

Evacuations from Mariupol and other besieged cities proceeded along eight of 10 humanitarian corridors that Ukraine and Russia agreed to on Saturday Deputy Prime Minister Iryna Vereshchuk said, and a total of 6,623 people left.

Vereshchuk said planned humanitarian aid for the southern city of Kherson, which Russia seized early in the war, could not be delivered because the trucks were stopped along the way by Russian troops.

Ukraine and Russia have held several rounds of negotiations aimed at ending the conflict but remain divided over several issues, with Moscow pressing for its neighbor’s demilitarization and Kyiv demanding security guarantees.

Around Ukraine, hospitals, schools and buildings where people sought safety have been attacked.

A satellite image from Maxar Technologies released Saturday confirmed earlier reports that much of the theater in Mariupol was destroyed. It also showed the word “CHILDREN” written in Russian in large white letters outside the building.

Russian forces fired on eight cities and villages in the eastern Donetsk region between Friday and Saturday, Ukraine’s national police said. Dozens of civilians were killed or wounded, and at least 37 residential buildings and facilities were damaged including a school, a museum and a shopping center.

In the western city of Lviv, Ukraine’s cultural capital, which was hit by Russian missiles on Friday, military veterans were training dozens of civilians on how to handle firearms and grenades.

“It’s hard, because I have really weak hands, but I can manage it,” said one trainee, 22-year-old Katarina Ishchenko.

The Mariupol city council claimed Saturday that Russian soldiers had forcibly relocated several thousand city residents, mostly women and children, to Russia. It didn’t say where, and AP could not immediately confirm the claim.

Zelenskyy adviser Oleksiy Arestovych said the nearest forces that could assist Mariupol were already struggling against “the overwhelming force of the enemy” and that “there is currently no military solution to Mariupol.”

Zelenskyy on Sunday ordered 11 political parties with links to Russia, the largest of which has 44 out of 450 seats in the country’s parliament, to suspend activities during the period of martial law.

“Activities by politicians aimed at discord and collaboration will not succeed,” he said in the address.

(AP)