Ukrainian officials defiantly rejected a Russian demand that their forces in Mariupol lay down their arms and raise white flags Monday in exchange for safe passage out of the besieged port city.

As Russia intensified its effort to pound Mariupol into submission, its ground offensive in other parts of Ukraine has become bogged down. Western officials and analysts say the conflict is turning into a grinding war of attrition, with Russia bombarding cities.

In the capital, Kyiv, a shopping center in the densely populated Podil district near the city center was a smoking ruin after being hit late Sunday by shelling that killed eight people, according to emergency officials. The attack shattered every window in a neighboring high-rise.

Ukrainian authorities also said Russia shelled a chemical plant in northeastern Ukraine, sending toxic ammonia leaking into the air, and hit a military training base in the west with cruise missiles.

The encircled southern city of Mariupol on the Sea of Azov has seen some of the worst horrors of the war, under Russian pounding for more than three weeks, in what Ukrainian and Western officials have branded a war crime.

Hours before Russia’s offer to open corridors out of the city in return for the capitulation of its defenders, an art school where some 400 people were taking shelter was hit by an airstrike, according to Ukrainian officials.

“They are under the rubble, and we don’t know how many of them have survived,” Ukraine’s President Volodymyr Zelenskyy said. In a video address, he vowed that Ukraine would “shoot down the pilot who dropped that bomb.”

Russian Col. Gen. Mikhail Mizintsev had offered two corridors — one heading east toward Russia, the other west to other parts of Ukraine — in return for Mariupol’s surrender. He did not say what Russia would do if the offer was rejected.

The Russian Ministry of Defense said authorities in Mariupol could face a military tribunal if they sided with what it described as “bandits,” the Russian state news agency RIA Novosti reported.

Ukrainian officials rejected the proposal even before Russia’s deadline of 5 a.m. Moscow time for a response came and went.

“There can be no talk of any surrender, laying down of arms,” Ukrainian Deputy Prime Minister Irina Vereshchuk told the news outlet Ukrainian Pravda.

The strike on the art school was the second time in less than a week that officials reported an attack on a public building where Mariupol residents had taken shelter. On Wednesday, an airstrike devastated a theater where more than 1,000 people were believed to be sheltering. At least 130 people were reported rescued Friday, but there has been no update since then.

Mariupol officials said at least 2,300 people have died in the siege, with some buried in mass graves.

City officials and aid groups say Russian bombardment has cut off Mariupol’s electricity, water and food supplies and severed its communications with the outside world, plunging the remaining residents into a chaotic fight for survival.

“What’s happening in Mariupol is a massive war crime,” EU foreign policy chief Josep Borrell said.

Mariupol had a prewar population of about 430,000. About a quarter were believed to have left in the opening days of the war, and tens of thousands got out over the past week by way of a humanitarian corridor, though other attempts have been thwarted by the bombardment.

In the Black Sea port city of Odesa, authorities said Russian forces damaged civilian houses in a strike Monday. The city council said no one was killed.

Russia’s invasion has driven nearly 3.4 million people from Ukraine, according to the United Nations. The U.N. has confirmed over 900 civilian deaths but said the actual toll is probably much higher. Estimates of Russian deaths vary, but even conservative figures are in the low thousands.

Some who were able to escape Mariupol tearfully hugged relatives as they arrived by train Sunday in Lviv in western Ukraine.

“Battles took place over every street. Every house became a target,” said Olga Nikitina, who was embraced by her brother as she got off the train. “Gunfire blew out the windows. The apartment was below freezing.”

Mariupol is a key Russian target because its fall would allow Russian forces in southern and eastern Ukraine to link up. Its capture would also help Russia establish a land bridge to Crimea, which was seized from Ukraine in 2014.

More than three weeks into the invasion, the two sides seem to be trying to wear each other down, experts say, with Russian forces launching long-range missiles at cities and military bases as Ukrainian forces carry out hit-and-run attacks.

U.S. Defense Secretary Lloyd Austin said Russian President Vladimir Putin’s forces on the ground are “essentially stalled.”

Talks between Russia and Ukraine have continued by video conference but failed to bridge the chasm between the two sides, with Russia demanding Ukraine disarm and declare itself neutral and Ukraine saying Russian forces must withdraw from the whole country.

Ukrainian delegation member Davyd Arakhamia told Ukrainska Pravda that there was a 90-minute session between top negotiators Monday morning, to be followed by a full day of talks in various working groups.

U.S. President Joe Biden was expected to talk Monday with the leaders of France, Germany, Italy and Britain about the war. The Russian Foreign Ministry warned that relations with the U.S. are “on the verge of a breach,” citing “unacceptable statements” by Biden about Putin — an apparent reference to the American calling the Russian a “war criminal.”

In Ukraine’s major cities, hundreds of men, women and children have been killed in Russian attacks.

Ukraine’s prosecutor general said a Russian shell struck a chemical plant outside the eastern city of Sumy just after 3 a.m. Monday, causing a leak in a 50-ton tank of ammonia that took hours to contain.

Russian military spokesman Igor Konashenkov claimed the leak was a “planned provocation” by Ukrainian forces to falsely accuse Russia of a chemical attack.

Konashenkov also said an overnight cruise missile strike hit a military training center in the Rivne region of western Ukraine. He said 80 foreign and Ukrainian troops were killed, though the figure could not be independently confirmed.

Britain’s defense ministry said that Ukrainian resistance had kept the bulk of Russian forces more than 25 kilometers (15 miles) from the city center, but that Kyiv “remains Russia’s primary military objective.”

Russian troops are shelling Kyiv for a fourth week now and are trying to surround the capital, which had nearly 3 million people before the war. Kyiv Mayor Vitali Klitschko announced a curfew extending from Monday evening through Wednesday morning.

A cluster of villages on Kyiv’s northwest edge, including Irpin and Bucha, have been all but cut off by Russian forces and are on the verge of humanitarian catastrophe, regional officials said. Associated Press journalists who were in the area a week ago saw bodies in a park.

In another worrying development, Ukraine’s nuclear regulatory agency said radiation monitors around the decommissioned Chernobyl power plant, the site in 1986 of the world’s worst nuclear meltdown, have stopped working.

The agency said that problem, and a lack of firefighters to protect the area’s radiation-tainted forests as the weather warms, could mean a “significant deterioration” in the ability to control the spread of radiation in Ukraine and beyond.

(AP)