LVIV, Ukraine (AP) — As Mariupol’s defenders held out Monday against Russian demands that they surrender, the number of bodies in the rubble of the bombarded and encircled Ukrainian city remained shrouded in uncertainty, the full extent of the horror not yet known.

With communications crippled, movement restricted and many residents in hiding, the fate of those inside an art school flattened on Sunday and a theatre that was blown apart four days earlier was unclear.

More than 1,300 people were believed to be sheltering in the theatre, and 400 were estimated to have been in the art school.

Perched on the Sea of Azov, Mariupol has been a key target that has been relentlessly pounded for more than three weeks and has seen some of the worst suffering of the war. The fall of the southern port city would help Russia establish a land bridge to Crimea, seized from Ukraine in 2014.

But no clear picture emerged how close its capture might be.

“Nobody can tell from the outside if it really is on the verge of being taken,” said Keir Giles, a Russia expert at the British think tank Chatham House.

Over the weekend, Moscow had offered safe passage out of Mariupol — one corridor leading east to Russia, another going west to other parts of Ukraine — in return for the city’s surrender before daybreak Monday. Ukraine flatly rejected the offer well before the deadline.

Mariupol officials said at least 2,300 people have died in the siege, with some buried in mass graves, but fears grew that the number could be far higher.

For those who remain, conditions have become brutal. The bombardment has cut off Mariupol’s electricity, water and food supplies and severed communication with the outside world, plunging residents into a fight for survival.

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

Mariupol had a prewar population of about 430,000. Around a quarter were believed to have left in the opening days of the war, and tens of thousands escaped over the past week by way of a humanitarian corridor. Other attempts have been thwarted by the fighting.

Those who have made it out told of a devastated city.

“There are no buildings there anymore,” said 77-year-old Maria Fiodorova, who crossed the border to Poland on Monday after five days of travel.

Olga Nikitina, who fled Mariupol for the western Ukrainian city of Lviv, where she arrived Sunday, said gunfire blew out her windows, and her apartment dropped below freezing.

“Battles took place over every street. Every house became a target,” she said.

As Russia intensifies its effort to pound Mariupol into submission, its ground offensive in other parts of the country has become bogged down, slowed by lethal hit-and-run attacks by the Ukrainians. Western officials and analysts say the conflict is turning into a grinding war of attrition, with Russian President Vladimir Putin’s forces using air power and artillery to pulverize cities from a distance.

In the capital, Kyiv, a shopping centre in the densely populated Podil district near the city centre 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 neighbouring high-rise.

Russian military spokesman Major General Igor Konashenkov charged that Ukrainian forces had been using the shopping mall to store rockets and reload launchers. That claim could not be independently verified.

Britain’s defence ministry said Ukrainian resistance has kept the bulk of Moscow’s forces more than 25 kilometres (15 miles) from the centre of Kyiv, but the capital “remains Russia’s primary military objective”.

Amid the continuing shelling, Kyiv Mayor Vitali Klitschko announced a curfew extending from Monday evening through Wednesday morning.

Ukrainian authorities also said Russia shelled a chemical plant outside the eastern city of Sumy, sending toxic ammonia leaking from a 50-ton tank, and hit a military training base in the Rivne region of western Ukraine with cruise missiles.

Konashenkov said 80 foreign and Ukrainian troops were killed in the Rivne attack. There was no immediate word from the Ukrainian side on casualties.

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 3.5 million people from Ukraine, according to the United Nations. The UN has confirmed over 900 civilian deaths but said the real toll is probably much higher. Estimates of Russian deaths vary, but even conservative figures are in the low thousands.

Talks between Russia and Ukraine have continued by video but failed to bridge the chasm between the two sides, with the Kremlin demanding Ukraine disarm and declare itself neutral and Ukraine calling for binding security guarantees and a withdrawal of all Russian forces.

Russia’s foreign ministry warned that relations with the US are “on the verge of a breach”, citing “unacceptable statements” by US President Joe Biden about Putin. Biden last week branded the Russian leader a war criminal.

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, and could mean a “significant deterioration” in the ability to control the spread of radiation in Ukraine and beyond.