‘Hellscape’ in Mariupol where 100,000 trapped

Almost 100,000 people are trapped among the ruins of the Ukrainian city of Mariupol, facing starvation, thirst and relentless Russian bombardment, President Volodymyr Zelensky said as the UN sharpened demands for Moscow to end its "absurd" and "unwinnable" war.

Tens of thousands of residents have already fled the besieged southern port city, bringing harrowing testimony of a "freezing hellscape riddled with dead bodies and destroyed buildings", according to Human Rights Watch.

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In his latest video address Tuesday, Zelensky said more than 7,000 people had escaped in the last 24 hours alone, but one group travelling along an agreed humanitarian route west of the city were "simply captured by the occupiers."

He warned that many thousands more were unable to leave as the humanitarian situation worsens.

"Today, the city still has nearly 100,000 people in inhumane conditions. In a total siege. Without food, water, medication, under constant shelling and under constant bombing," he said, renewing calls for Russia to allow safe humanitarian corridors for civilians to escape.

Refugees wait for Ukrainian police to check their papers and belongings in Brovary, Ukraine. AP

Satellite images of Mariupol released by private company Maxar showed a charred landscape, with several buildings ablaze and smoke billowing from the city.

The Pentagon has said Russia is now pummelling Mariupol using artillery, long-range missiles and from naval ships deployed in the nearby Sea of Azov.

Local Ukrainian forces also report "heavy" ground fighting with Russian "infantry storming the city" after they rejected a Monday ultimatum to surrender.

UN relief agencies estimate there have been around 20,000 civilian casualties in the city, and perhaps 3,000 killed, but they stress "the actual figure remains unknown."

Mariupol's former mayor Sergiy Taruta vowed the city would never forgive Russia's siege.

This satellite image shows burning and destroyed apartment buildings in Mariupol, Ukraine, on Tuesday. AP

"There will never be enough rage. There will never be enough revenge. There will never be enough of retribution," he said in a Facebook post.

"For all the lives taken, the fates broken, for all the children killed, tears and suffering, each of the occupiers will never be at peace."

The almost month-long siege of Mariupol has brought ever-harsher international condemnation.

United Nations chief Antonio Guterres on Tuesday called for Russia to end its "absurd war."

"Even if Mariupol falls, Ukraine cannot be conquered city by city, street by street, house by house," he said.

"This war is unwinnable. Sooner or later, it will have to move from the battlefield to the peace table. That is inevitable."

Mariupol is a pivotal target in President Vladimir Putin's war -- providing a land bridge between Russian forces in Crimea to the southwest and Russian-controlled territory to the north and east.

As US President Joe Biden readied to visit allies in Europe, he warned that with Russia's offensive stalling, Putin was considering using chemical and biological weapons.

"Now Putin's back is against the wall," Biden said. "And the more his back is against the wall, the greater the severity of the tactics he may employ."

Multiple explosions and rising smoke are seen around an industrial compound in Mariupol. Reuters

Kremlin spokesman Dmitry Peskov has stated that Russia would use nuclear weapons if under "existential threat."

Biden is due to travel to Brussels on Thursday for a series of summits gathering NATO, EU and G7 leaders, before heading to Poland, which has received the bulk of more than 3.5 million Ukrainians fleeing war in their country.

On the ground, Russia's defence ministry has reported some advances in the southeast of Ukraine and boasted of strikes against "military infrastructure" across the country.

But Ukraine and its allies have claimed Russian forces are severely depleted, poorly supplied and still unable to carry out complex operations.

In the face of intense Ukrainian resistance, the Pentagon believes as much as 10 per cent of Russian forces committed to Ukraine may have been knocked out of the war in just four weeks of fighting.

Ukrainian President Volodymyr Zelenskyy appears on the television inside a inn in Lyiv, Ukraine, Tuesday. AP

"The Russians may be slightly below a 90 per cent level of assessed available combat power," a senior defence official told reporters in Washington, adding that some Russian forces were suffering from frostbite.

Ukraine's army command said it believes Russian troops now had enough ammunition, food and fuel to last just three days and there are reports of hundreds of Russian soldiers defecting.

For the first time, there are signs that Ukrainian forces are going on the offensive, retaking a town near Kyiv and attacking Russian forces in the south of the country.

In the southern city of Mykolaiv, one bulwark of the fierce fightback, residents said they were determined to stay and defend it despite incessant bombardment.

At the burial of soldier Igor Dundukov, 46, his brother Sergei wept as he kissed his sibling's swollen, blood-stained face.

"We supported his commitment to defending our homeland," Sergei told AFP. "This is our land. We live here. Where would we run to? We grew up here."

And even in areas Russia has captured, resistance has persisted.

In the occupied southern city of Kherson, Ukraine's leaders on Tuesday accused Russian troops of firing on unarmed protesters.

Videos posted on social media and the messaging app Telegram showed citizens gathering in Kherson's "Freedom Square" protesting against Russia's recent seizure of the city.

Russian soldiers could be seen firing into the air.

Amid the bloodshed, Moscow and Kyiv have begun holding peace negotiations remotely after in-person talks between delegations meeting on the border of Belarus and Ukraine made little progress.

Russia has said it wants "more substantial" discussion and Zelensky has said all issues would be on the table if Putin agreed to direct talks.

"We continue to work at various levels and to push Russia for peace," the Ukrainian leader said late Tuesday. "It's very difficult. Sometimes scandalous. But step by step we are moving forward."

Agence France-Presse