Ukrainian officials rejected a Russian demand that their forces in Mariupol surrender Monday in exchange for safe passage out of the port city.

Russia has intensified its efforts to force the surrender of Mariupol. But the Russian ground offensive in other parts of Ukraine has failed to move forward. Western officials and experts say the conflict is turning into a war of attrition, with Russia bombarding cities.

In the capital Kyiv, a shopping center in the Podil neighborhood near the city center was a smoking ruin after being hit late Sunday. The shelling killed eight people, said emergency officials.

Ukrainian officials also said Russia shelled a chemical production center in northeastern Ukraine, sending toxic ammonia into the air. Russian forces also hit a military training base in the west with cruise missiles.

The surrounded southern city of Mariupol has seen some of the worst horrors of the war. It has been under Russian attack for more than three weeks. Ukrainian and Western officials have called the Russian attacks on the city a war crime.

Russia offered to open the city so people could escape in exchange for surrender. Hours before the offer came, Ukrainian officials said an art school where some 400 people were taking shelter was hit by an airstrike.

"They are under the rubble, and we don't know how many of them have survived," Ukraine's President Volodymyr Zelenskyy said.

Russian Colonel-General Mikhail Mizintsev had offered two escape paths — 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 officials in Mariupol could face a military tribunal if they sided with what it described as "bandits," the Russian state news agency RIA Novosti reported.

A tribunal is a person or group given the power to serve as a court.

Ukrainian officials rejected the proposal before Russia's deadline.

"There can be no talk of any surrender, laying down of arms," Ukrainian Deputy Prime Minister Irina Vereshchuk told 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 destroyed a theater where more than 1,000 people were believed to be sheltering.

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

City officials and aid groups said Russian bombardment has cut off Mariupol's electricity, water and food supplies. It also has cut off its communications with the outside world.

"What's happening in Mariupol is a massive war crime," EU foreign policy chief Josep Borrell said.

Mariupol had a population of about 430,000 before the war. About 25 percent were believed to have left in the opening days of the war. Tens of thousands fled over the past week through a humanitarian escape path.

The United Nations said Russia's invasion has driven nearly 3.4 million people from Ukraine. The U.N. has confirmed over 900 civilian deaths but said the real number is probably much higher. There are many estimates of the number of Russian deaths, but most are in the low thousands.

Some who were able to escape Mariupol met 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 met her brother as she got off the train.

Mariupol is an important target for Russia. Its capture would let Russian forces in southern and eastern Ukraine join together. It would also help Russia establish a land bridge to Crimea, which was taken from Ukraine in 2014.

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

Talks between Russia and Ukraine have continued by video link. But they have failed to produce agreement. Russia is demanding Ukraine disarm and declare itself neutral. Ukraine is saying Russian forces must withdraw from the whole country.

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 nearing "a breach." Putin is reportedly angry that Biden called him a "war criminal."

I'm Susan Shand.

The Associated Press reported this story. Susan Shand adapted it for Learning English.

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attrition – n. the act or process of weakening and gradually defeating an enemy through constant attacks and continued pressure over a long period of time

toxic – adj. containing poisonous substances

rubble – n. broken pieces of stone, or brick from walls or buildings that have fallen

bandit – n. a criminal who attacks and steals from travelers and who is often a member of a group of criminals

siege – n. a situation in which soldiers or police officers surround a city or building in order to try to take control of it