Ukraine rejected a Russian ultimatum to surrender the besieged southern city of Mariupol Monday, as renewed overnight shelling killed six in the capital Kyiv.
The move came as US President Joe Biden announced a trip to Poland for crisis talks over the Russian invasion, AFP said.
Ukraine's Deputy Prime Minister Iryna Vereshchuk dismissed the early morning deadline, saying Moscow should instead allow hundreds of thousands of trapped Mariupol residents to escape.
"We can't talk about surrendering weapons," Vereshchuk told the Ukrainska Pravda online newspaper, "we have already informed the Russian side about it."
The Kremlin's military command had warned authorities in Mariupol had until "5am tomorrow, that is on March 21" to respond to eight pages of demands, which Ukrainian officials said would amount to a capitulation.
"We call on units of the Armed Forces of Ukraine, territorial defense battalions, foreign mercenaries to stop hostilities, lay down their arms" said Russian Colonel-General Mikhail Mizintsev, warning those who did not surrender would face court martial, and worse.
Kyiv's rejection came as Russian bombs hit targets across the country overnight, killing at least six in Kyiv and allegedly damaging a chemical plant in the north of the country causing an "ammonia leakage".
Sumy regional governor Dmytro Zhyvytsky said "Russian artillery shelling" had hit the Sumykhimprom fertilizer plant as he warned residents within a 2.5 kilometers (1.5 miles) radius to seek shelter.
In Washington, the White House said Biden -- who is due to visit Europe this week to meet leaders from NATO, the G7 and the European Union -- would also travel to Poland.
There he is expected to hold talks with President Andrzej Duda to discuss a joint response to the humanitarian crisis that has seen around two million Ukrainians flee to Poland alone.
The humanitarian crisis is perhaps most acute in Mariupol, where for almost a month Russian forces have bombarded and besieged the southern port city trapping an estimated 350,000 people.
The UN has described the humanitarian situation in the city as "extremely dire" with "residents facing a critical and potentially fatal shortage of food, water and medicines".
Mariupol is a pivotal target in Vladimir Putin's war in Ukraine -- providing a land bridge between Russian forces in Crimea to the southwest and Russian-controlled territory to the north and east.
A Greek diplomat who remained in the city during some of the bombardment said it would rank alongside history's most ruinous wartime assaults.
"Mariupol will be included in a list of cities in the world that were completely destroyed by the war, such as Guernica, Stalingrad, Grozny, Aleppo," Manolis Androulakis said after flying back to Athens.
- 'Act of terror' -
In his latest video address Ukrainian President Volodymyr Zelensky accused Russia of bombarding a Mariupol school sheltering hundreds, calling it an act of "terror that will be remembered even in the next century".
"Russian forces have come to exterminate us, to kill us," he said.
It was the latest potentially devastating strike on a shelter for civilians. Last Wednesday, a theatre where authorities said more than 1,000 people had sheltered was hit, with hundreds still presumed missing in the rubble.
Mariupol officials have said occupying forces have forcibly transported around a thousand residents to Russia and stripped them of their Ukrainian passports -- a possible war crime.
A group of children stuck in a Mariupol clinic for weeks are among those who have been taken to Russian-controlled territory, a carer and a relative of a clinic worker told AFP.
The 19 children, aged between four and 17 and mostly orphans, had been living in freezing cellars hiding from shelling in harrowing conditions.
- 'Perhaps in Jerusalem' -
Amid the carnage, Zelensky has again suggested he and Putin hold direct talks.
After addressing Israeli lawmakers Zelensky -- who is accused by Russia of being a Nazi, but is Jewish -- thanked Prime Minister Naftali Bennett for efforts to broker talks, which he suggested could take place in Jerusalem.
"Sooner or later we could start the conversation with Russia. Perhaps in Jerusalem. This is the right place for finding peace. If this is possible," Zelensky said.
Authorities in Turkey, where Russian and Ukrainian representatives have been negotiating, said the two sides were close to a deal to stop the fighting.
But the Ukrainian leader appeared to draw some red lines.
"You cannot just demand from Ukraine to recognize some territories as independent republics," he told CNN. "We have to come up with a model where Ukraine will not lose its sovereignty."
- Hypersonic missiles -
As the war has ground on, Russian forces -- stymied by unexpectedly fierce Ukrainian resistance, and reportedly facing shortages of weapons and supplies -- have made increasing use of long-range missiles.
Russia's defense ministry said Sunday that Moscow had again fired its newest Kinzhal (Dagger) hypersonic missile, destroying a fuel depot in the southern Mykolaiv region.
A day earlier, Russia said it had used the sophisticated weapon to destroy an arms depot near Ukraine's border with Romania.
The Pentagon, however, played down the claim.
Humanitarian conditions continued to deteriorate in the mostly Russian-speaking south and east, where Russian forces have been pressing their advance, as well as in the north around Kyiv.
Aid agencies are struggling to reach people trapped in besieged cities.
Around 10 million Ukrainians have fled their homes, roughly one-third going abroad, the UN refugee agency said.
They are fleeing fighting that, according to Zelensky, has left around 14,000 Russian servicemen dead, a number that "will only continue to rise".
Russia has provided no death toll since early March, when it said nearly 500 servicemen had been killed. Ukrainian officials said on March 12 that some 1,300 Ukrainian troops had died.
Ukraine has not been providing a civilian toll, except for children, saying at least 115 have now perished.
- Lasting economic effects -
Russia's war has sparked an unprecedented wave of Western sanctions against Putin, his entourage and Russian companies.
France said Sunday it had seized 850 million euros ($920 million) of Russian oligarchs' assets on its soil.
The war has sparked turmoil for an already vulnerable world economy. Russia is a major exporter of oil, gas and commodities, while Ukraine is a major supplier of wheat.
Commodity prices have rocketed, further fueling high inflation, the chief economist with the European Bank for Reconstruction and Development told AFP.
"Even if the war stopped today, the consequences of this conflict would be felt for months to come," Beata Javorcik said.