KYIV, March 21 - Ukraine rejected demands from Moscow on Monday to surrender the strategically important port city of Mariupol, as Russian forces continued relentless attacks aimed at forcing the city into submission, NBC News reported.

Shortly after Russia said that Ukraine had until 5 a.m. Monday to give up Mariupol in exchange for safe passage out of the city, Russian shelling hit an art school where civilians were sheltering, the second strike on a public building in less than a week. A theater in Mariupol where more than 1,000 people were sheltering was leveled in an attack Wednesday. A maternity hospital in the city was also shelled March 9.

Mariupol has been subjected to weeks of shelling and grisly urban warfare as Russian forces have struggled to take over the city.

Mariupol is considered a key strategic target for Russia because it would create a bridge between Crimea — which Russia annexed by force in 2014 — and areas of eastern Ukraine held by Moscow-backed separatists, making it easier for Russia to move people and supplies into the country.

Mariupol is also a steel and iron industrial center and home to one of Ukraine’s biggest ports. If Russia takes control, it would allow Moscow to further choke Ukraine's resources.

Ben Barry, a senior fellow for land warfare at the International Institute for Strategic Studies in London and a former director of the British army staff in the ministry of defense, said that if Mariupol falls, Russia could push in two directions.

"It is quite possible that once Mariupol falls, that will release Russian forces from the two republics to either push west to Odesa or indeed push north in an effort to threaten the rear of the Ukrainian forces operating in the Donbas, which would pose the Ukrainian high command with a difficult choice about whether it continued to fight there or whether it seeks to withdraw," he said.

Keir Giles, research director of the Conflict Studies Research Centre in London, also said that Mariupol was tying down significant numbers of Russian forces who could be released to other operations.

"The sad fact is when people sheltering in the theater in Mariupol write the word 'children' on either side of the building in letters big enough to be seen by Russian surveillance assets, they are actually making the building a target instead of protecting it," he said.

In a video posted on Telegram on Monday, President Volodymyr Zelenskyy said that around 400 people, mostly women, children and the elderly, were in the art school that was hit.

“They are under the rubble. We currently do not know how many survived,” he said.

Petro Andrushchenko, an adviser to Mariupol’s mayor, said in an interview with NPR on Monday that it is difficult to even dig people out of the rubble because Russian attacks are so frequent. "Every minute we have a Russian attack," he said. The city has been under fire for more than two weeks.

It is becoming increasingly difficult to verify information coming out of Mariupol. Few journalists remain on the ground because it is too dangerous.

City officials and aid groups have said that food, water and electricity are running low and the fighting has made it difficult for civilians to access humanitarian corridors out of the city.

Several times in past weeks, efforts to evacuate civilians along established routes were suspended when Russian and Ukrainian forces violated cease-fire agreements.

Ukrainian officials say more than 2,000 people have died in Mariupol. (nbc/ez)