'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.

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.

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.

"There will never be enough rage. There will never be enough revenge. There will never be enough 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.   

"But then there are just like little pieces that when I actually tee it up on Thursday that I'm missing from when I compare it to a really good round or a really good tournament.

"It's so frustrating because I feel like I can hit all those shots but then they're just not put together."