Ukraine rejects ultimatums as conflict intensifies

* Biden says Russia considering using biological weapons

* 8 dead as shelling wrecks Kyiv shopping centre

MARIUPOL/LVIV/KYIV: Ukraine said on Monday it would not obey ultimatums from Russia after Moscow demanded it stop defending besieged Mariupol, where hundreds of thousands of civilians are suffering through Russian bombardments laying waste to their city.

Mariupol has become a focal point of Russia's attack on Ukraine, but attacks were also reported to have intensified on the country's second city Kharkiv on Monday.

The conflict has driven almost a quarter of Ukraine's 44 million people from their homes, and Germany predicted the refugee number could reach as high as 10 million in coming weeks. Europe said Russia was using refugees as a tool and that it was prepared to take more action on top of existing sanctions to isolate Russia from global finances and trade.

Russia's military had ordered residents of Mariupol to surrender by 5 am local time on Monday, saying those who did so could leave, while those who stayed would be handed to tribunals run by Moscow-backed separatists.

President Volodymyr Zelenskiy's government responded that it would never bow to ultimatums and said cities such as the capital Kyiv, Mariupol and Kharkiv would always defy occupation.

"There can be no question of any surrender" in Mariupol, said Ukrainian Deputy Prime Minister Iryna Vereshchuk.

Meanwhile, Ukraine's military warned the public on Tuesday of more indiscriminate Russian shelling from bogged-down Russian troops, and US President Joe Biden issued his strongest warning yet that Russia is considering using chemical weapons.

Amid the devastation caused by Russia's unceasing bombardment of Ukrainian cities, President Zelenskiy highlighted the death of a 96-year-old survivor of Nazi concentration camps, killed in his flat by shelling in Kharkiv. Nearly four weeks into their attack, Russian troops have failed to capture any major Ukrainian city and have been halted on nearly all fronts, but are hammering residential districts with artillery, missiles and air strikes.

Russian forces were expected to continue to attack critical infrastructure with "high-precision weapons and indiscriminate munitions”, Ukraine's armed forces said in a statement.

Russia has been saying in recent weeks that Ukraine might possess chemical or biological weapons. Biden said those accusations were not merely false, but a sign that President Vladimir Putin might be planning to use such weapons himself.

Russia's attack, now in its fourth week, has largely stalled, failing to capture any major city, but causing massive destruction to residential areas.

Mariupol, a port city on the Sea of Azov that was home to 400,000 people, has run short of food, medicine, power and water. Ukrainian Defence Minister Oleksii Reznikov said its "heroic defenders" had helped thwart Russia elsewhere.

A part of Mariupol now held by Russian forces, reached by Reuters on Sunday, was an eerie wasteland. Several bodies lay by the road, wrapped in blankets. Windows were blasted out and walls were charred black. People who came out of basements sat on benches amid the debris, bundled up in coats.

Some, though, are managing to escape. A total of 8,057 people were safely evacuated on Monday through seven humanitarian corridors from towns and cities under fire, said Vereshchuk. Among those brought to safety were 3,007 residents of Mariupol.

Russia calls the war, the biggest attack on a European state since World War Two, a "special military operation" to disarm Ukraine and protect it from "Nazis".

The West calls this a false pretext for an unprovoked war of aggression by President Vladimir Putin. President Joe Biden told businesses to be alert for possible cyber attacks by Russia and do more to protect themselves. "It’s part of Russia’s playbook," he said in a statement.

"My administration is reiterating those warnings based on evolving intelligence that the Russian government is exploring options for potential cyberattacks."

There was no immediate reply from the Kremlin to a request for comment from Reuters out of hours on Monday.

Russia has previously rejected similar allegations including that it was responsible for hacks on Ukrainian banking and government websites in February. - Reuters