Kyiv (AFP) - Ukraine appealed for more Western military help ahead of a NATO summit, as it warned that almost 100,000 people are trapped by Russian bombardment and facing starvation in the ruins of the besieged port of Mariupol.
Tens of thousands of residents have already fled the southern city, bringing harrowing testimony of a "freezing hellscape riddled with dead bodies and destroyed buildings", according to Human Rights Watch.
As the UN demanded Russia end its "unwinnable" war, Ukrainian President Volodymyr Zelensky told the Japanese parliament that the UN Security Council was dysfunctional and in need of reform, after Russia wielded its veto to nix condemnation of its invasion.
Nearly a month on, peace talks have agreed on daily humanitarian corridors for refugees, and Ukraine says it is willing to countenance some Russian demands subject to a national referendum.
But it has refused to bow to Russian pressure to disarm and renounce all Western alliances, and Zelensky was also due Thursday to address a NATO meeting in Brussels joined by US President Joe Biden.
Ukraine's lead negotiator Mykhaylo Podolyak said the peace talks were encountering "significant difficulties", after Moscow accused the United States of undermining the process.
Russia meanwhile refuses to rule out using nuclear weapons if it faces an "existential threat", Kremlin spokesman Dmitry Peskov told CNN.
Pentagon spokesman John Kirby slammed Moscow's "dangerous" rhetoric, and Biden warned en route to Europe that Russia may also use chemical weapons in Ukraine as its ground offensive stalls.
"Our armed forces and citizens are holding out with superhuman courage," Andriy Yermak, a top advisor to Zelensky, said as Biden travelled to the summits of NATO, the G7 and European Union in Brussels.
"But we cannot win a war without offensive weapons, without medium-range missiles that can be a means of deterrence," Yermak said.
NATO chief Jens Stoltenberg said the leaders at Thursday's summit would agree to "major increases of forces" including four new battle groups in Bulgaria, Hungary, Romania and Slovakia.
The allies will also offer "additional support" to Ukraine against nuclear and chemical threats, he said.
Charred landscape
For Ukrainians besieged in Mariupol and other cities, Russian talk of peace rings hollow as they come under indiscriminate shelling that Western countries say amounts to a war crime.
In a video address, Zelensky said more than 7,000 people had escaped Mariupol in the last 24 hours, but one group travelling along an agreed humanitarian route west of the city was "simply captured by the occupiers".
"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.
Satellite images of Mariupol released by private company Maxar showed a charred landscape, with several buildings ablaze and smoke billowing from the city.
Ukrainian forces also reported "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 Mariupol, and perhaps 3,000 killed, but they stress the actual figure remains unknown.
"Even if Mariupol falls, Ukraine cannot be conquered city by city, street by street, house by house," United Nations chief Antonio Guterres 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.
Putin threatens 'Russia's future'
"Putin's offensive is stuck despite all the destruction that it is bringing day after day," German Chancellor Olaf Scholz said in a speech to the Bundestag, warning of further Western sanctions against Russia.
Putin "must hear the truth" that not only is the war destroying Ukraine, "but also Russia's future", he said.
After Brussels, Biden will head on to Poland, which has received the bulk of more than 3.6 million Ukrainians fleeing the war.
The president will consult with allies on new sanctions and on potentially throwing Russia out of the G20, US officials said.
"We believe that it cannot be business as usual for Russia in international institutions and in the international community," White House National Security Advisor Jake Sullivan told reporters.
China, a leading member of the G20, pushed back against expelling Russia from the group of major economies, and Moscow said Putin still intended to join its November summit in Indonesia.
On the ground, Russia's defence ministry has reported some advances in the southeast of Ukraine and boasted of strikes using next-generation weaponry 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.
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.
'Morale is high'
In the southern city of Mykolaiv, one bulwark of the fightback, residents said they were determined to stay 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."
In the capital Kyiv, a 35-hour curfew ended early Wednesday 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.
Maxim Kostetskyi, 29, a lawyer, said residents had used the curfew 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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