US President Joe Biden said on Monday that it’s "clear" Russia is considering the use of chemical and biological weapons in Ukraine and warned of a "severe" Western response if it chose to do so.
It comes as Ukrainian President Volodymr Zelenskyy has said all issues would be on the table if his Russian counterpart Vladimir Putin agreed to direct talks to end the war, but he warned his country would be "destroyed" before it surrenders.
On the ground, there was no let-up in the violence, with Kyiv under a new 35-hour curfew after Russian strikes reduced a Kyiv shopping mall to rubble, and the Pentagon saying Moscow was stepping up air and sea operations.
Nearly a month into the conflict, there has been little progress in talks between the two sides, and Mr Zelenskyy has repeatedly urged direct discussions with his Russian counterpart.
He insisted again on Monday that a meeting with Mr Putin "in any format" was needed to end the war.
"If I have this opportunity and Russia has the desire, we would go through all the questions," he told Ukrainian journalists in an interview published by media outlet Suspilne.
"Would we solve them all? No. But there is chance, that we partially could — at least to stop the war," he added.
Mr Zelenskyy said he was even willing to discuss Russian-occupied Crimea and the breakaway Russian regions in Donbas, though he insisted he still believes they must be returned to Ukraine.
"At the first meeting with the president of Russia, I am ready to raise these issues," he said, adding that any agreement involving "historic" changes would be put to a national referendum.
"This is a very difficult story for everyone. Crimea and Donbas... And to find a way out, we need to take this first step, which I spoke about: security guarantees, the end of the war," he added.
He repeated his assertion that Ukraine "already understood" it could not join NATO but he added that his countrymen would not simply "hand over" the capital, the eastern city of Kharkiv, or the heavily bombarded and besieged southern port of Mariupol.
"Ukraine cannot fulfil Russian ultimatums," he said. "We should be destroyed first."
Meanwhile, Ukraine's military said on Tuesday residents should brace for more indiscriminate Russian shelling of critical infrastructure.
The country's armed forces said in a statement that Russian forces were expected to continue to attack critical infrastructure using "high-precision weapons and indiscriminate munitions”.
Kyiv curfew
In Kyiv, a 35-hour curfew came into effect from 8pm on Monday (Tuesday, 5am AEDT), after Russian strikes laid waste to the Retroville shopping complex, killing at least eight people.
"It's the biggest bomb to have hit the city until now," said 30-year-old Dima Stepanienko, who found himself flung to "the foot of his bed" by the explosion.
An Orthodox priest walking through the wreckage whispered prayers while cursing "Russian terrorists".
Russia claimed the mall was being used to store rocket systems and ammunition.
A woman measures a window before covering it with plastic sheets in a building damaged by a bombing the previous day in Kyiv, Ukraine, on Monday, 21 March, 2022. Source: AAP, AP / Vadim Ghirda
Without citing evidence, Mr Biden said Russia's false accusations that Kyiv had biological and chemical weapons illustrated that Mr Putin was considering using them himself.
Mr Putin's "back is against the wall and now he's talking about new false flags he's setting up including, asserting that we in America have biological as well as chemical weapons in Europe, simply not true," Mr Biden said at a Business Roundtable event.
"They are also suggesting that Ukraine has biological and chemical weapons in Ukraine. That's a clear sign he's considering using both of those," he said.
Mr Biden also told businesses to be alert for possible cyber attacks by Russia.
"It's part of Russia's playbook," he said in a statement.
Mr Biden is due to travel to Europe this week for meetings with allied leaders to discuss tighter sanctions on Russia, on top of the unprecedented financial penalties already announced.
Ahead of the trip he discussed Russia's "brutal" tactics in a call with European leaders on Monday, the White House said.
Mr Putin's troops have already been accused of causing a humanitarian catastrophe in Mariupol, where some 350,000 people are trapped without water and electricity.
The city has been relentlessly assaulted, with more than 2,000 people killed, according to local officials, in what European Union foreign policy chief Josep Borrell described as a "massive war crime".
Russia had given Mariupol until 5am on Monday (2pm AEDT) to surrender, but Kyiv rejected the ultimatum and said the city's resistance was bolstering the defence of all of Ukraine.
Mariupol is a pivotal target in Mr Putin's war — providing a land bridge between Russian forces in Crimea to the southwest and Russian-controlled territory to the north and east.
A mother embraces her son who escaped the besieged city of Mariupol and arrived at the train station in Lviv, western Ukraine on Sunday, 20 March, 2022. Source: AAP, AP / Bernat Armangue
Kherson protests
But even in areas Russia has captured, resistance has continued, with Ukraine's leaders on Tuesday accusing Russian troops of firing on unarmed protesters in the occupied southern city of Kherson
A series of 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, and video footage showing a bleeding elderly man being carried away, though local officials said there were no fatalities.
Away from the frontlines, diplomatic manoeuvring will continue this week with Mr Biden flying to Europe for talks with G7, EU and NATO leaders.
Western countries and allies are expected to shore up their united front, though possible additional steps to pressure Russia remain unclear.
Moscow has warned relations with Washington are "on the verge of rupture," after Mr Biden branded Mr Putin a "war criminal".
CCTV footage appears to show Russian troops (top, near building) firing stun grenades into a crowd of protesters, some with Ukrainian flags, amid the Russian invasion, along Ushakova Avenue in Kherson, Ukraine 21 March, 2022. Source: Reuters
Mr Zelenskyy has urged Europe to significantly dial up pressure on Moscow, saying the continent must cease all trade with Russia, but Germany has pushed back on a call for an EU embargo on Russian oil and gas.
At the Kremlin, spokesman Dmitry Peskov warned such an embargo would "have a very serious impact on the world energy market," where oil prices are already sky-high.
With the war edging closer to a stalemate, Mr Zelenskyy has also appealed to China, urging it to "play an important role in" ending the conflict.
The war has displaced around 10 million Ukrainians, with around a third becoming refugees, according to the UN, and sparked fears of famine elsewhere because Russia and Ukraine are both major agricultural exporters.
