Kyiv (AFP) - 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.
'We live here'
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."
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.
White House National Security Advisor Jake Sullivan said the United States would consult with allies about Russia's G20 membership.
"We believe that it cannot be business as usual for Russia in international institutions and in the international community," he told a press briefing Tuesday.
Washington has also not observed any Chinese arms shipments to Russia since Biden held a call with President Xi Jinping last week in which he raised concerns about Beijing's support for Moscow, Sullivan said.
"We have not seen...the provision of military equipment by China to Russia.But of course, this is something we are monitoring closely," he said.
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 percent of Russian forces committed to Ukraine may have been knocked out of the war in just four weeks of fighting.
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."
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.
More sanctions
Since Russia launched its invasion on February 24, at least 117 children have been killed in the war, Ukraine's federal prosecutor said.
Some 548 schools have been damaged and 72 destroyed.
Russia has pushed on with its assaults, in the face of unprecedented Western sanctions that have led international companies to pull out of the country.
More sanctions against Russia and tightening of existing measures will be announced Thursday when Biden meets European allies in Brussels.
In the capital Kyiv, a 35-hour curfew came into effect from Monday evening after Russian strikes laid waste to the Retroville shopping complex, killing at least eight people.
Russia claimed the mall was being used to store rocket systems and ammunition.
With businesses closed and residents told to stay home, Kyiv was a ghost town Tuesday, with air sirens and the distant sound of explosions regularly punching through the silence.
Maxim Kostetskyi, 29, a lawyer, said residents were using the "pause" imposed by the lockdown to regroup.
"We don't know if the Russians will continue with their efforts to encircle the city, but we are much more confident, the morale is high and inspiring," he told AFP.
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