KYIV (AFP) - Almost 100,000 people are trapped by Russian bombardment and facing starvation in the ruins of Mariupol, Ukraine's leader said, as Moscow accused Washington of undermining peace talks.

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.

As the UN demanded Russia end its "absurd" and "unwinnable" war, Ukrainian President Volodymyr Zelensky was on Wednesday (March 23) delivering a message of defiance to the Japanese and French parliaments.

Nearly a month on since Russia invaded Ukraine, stop-start 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 Mr Zelensky was also due on Thursday to address a Nato summit in Brussels joined by US President Joe Biden.

"The talks are tough, the Ukrainian side constantly changes its position," Russian Foreign Minister Sergey Lavrov said on Wednesday.

"It's hard to avoid the impression that our American colleagues are holding their hand," he said, claiming that Washington "apparently wants to keep us in a state of military action as long as possible".

Russia meanwhile refuses to rule out using nuclear weapons if it were facing an "existential threat", Kremlin spokesman Dmitry Peskov told CNN.

Pentagon spokesman John Kirby slammed Moscow's "dangerous" rhetoric, and Mr Biden warned that Russia may also use chemical and biological weapons in Ukraine as its ground offensive stalls.

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.

"Failing in their war against the Ukrainian people, the enemy is executing the total destruction of critical infrastructure," Ukraine's armed forces command said on Facebook.

In his latest video address, Mr 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 were "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 point out that 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.

Mr Putin "must hear the truth" that not only is the war destroying Ukraine, "but also Russia's future", he said.

Along with Nato, Mr Biden was also due to attend EU and G-7 summits on Thursday before heading to Poland, which has received the bulk of more than 3.5 million Ukrainians fleeing the war.

The President will consult with allies on new sanctions, and on potentially throwing Russia out of the G-20, 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 Adviser Jake Sullivan told reporters.

Washington has also not observed any Chinese arms shipments to Russia since Mr Biden held a call with President Xi Jinping last week in which he raised concerns about Beijing's support for Moscow, Mr Sullivan said.

China, a leading member of the G-20, pushed back against expelling Russia from the group of major economies.

"The G-20 is the main forum for international economic cooperation," Foreign Ministry spokesman Wang Wenbin said.

"Russia is an important member, and no member has the right to expel another country."

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 on 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.

Mr 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.