Ukrainian troops have accused Russian forces of bombing a hospital sheltering 300 people in the besieged port city of Mariupol. 

The deputy commander of the Azov regiment, who was among the troops remaining in Mariupol, said the Russian military dropped heavy bombs on the steel plant and hit an “improvised” hospital.

Serhiy Taruta, the former governor of the Donetsk region and a Mariupol native, also reported the bombing of the hospital, where he said 300 people, including wounded troops and civilians with children, were sheltered.

The reports could not be independently confirmed.

The eastern cities of Kharkiv and Kramatorsk also came under deadly attack. Russia also said it struck areas around Zaporizhzhia and Dnipro west of the Donbas with missiles.

Ukrainian President Volodymyr Zelensky said the Russian military was throwing everything it has into the battle, with most of its combat-ready forces now concentrated in Ukraine and just across the border in Russia.

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“They have driven almost everyone and everything that is capable of fighting us against Ukraine,” he said in his nightly video address to the nation on Tuesday.

Despite claims that they are hitting only military sites, the Russians continue to target residential areas and kill civilians, he said.

“The Russian army in this war is writing itself into world history forever as the most barbaric and inhuman army in the world,” Mr Zelensky said.

Weeks ago, after the abortive Russian push to take Kyiv, the Kremlin declared that its main goal was the capture of the mostly Russian-speaking Donbas, where Moscow-backed separatists have been fighting Ukrainian forces for eight years.

A Russian victory in the Donbas would deprive Ukraine of the industrial assets concentrated there, including mines, metals plants and heavy-equipment factories.

Military experts said the Russians’ goal is to encircle Ukrainian troops from the north, south and east.

Key to the campaign is the capture of Mariupol, which would deprive Ukraine of a vital port and complete a land bridge between Russia and the Crimean Peninsula, seized from Ukraine in 2014. It would also free up Russian troops to move elsewhere in the Donbas.

A few thousand Ukrainian troops, by the Russians’ estimate, remained holed up in a sprawling Mariupol steel plant, representing what was believed to be the last major pocket of resistance in the city.

Russia issued a new ultimatum to the Ukrainian defenders to surrender on Wednesday after a previous ultimatum was ignored. The Russian Defence Ministry said those who surrender will be allowed to live and given medical treatment. There was no immediate response from the Ukrainian troops, but they have repeatedly vowed not to give up.

Ukraine said later that its evacuation efforts to bring some civilians out of war-torn Mariupol will resume on Wednesday.

Deputy prime minister Iryna Vereshchuk said there is a “preliminary” agreement to operate a so-called humanitarian corridor westward to the Ukraine-controlled city of Zaporizhzhia.

It will apply to women, children and older people from Wednesday afternoon, she said.