US President Joe Biden has issued one of his strongest warnings yet that Russia is considering using chemical weapons in its invasion of Ukraine.
Russian troops have failed to capture any major Ukrainian city, more than four weeks into their invasion, and increasingly are resorting to massive destruction of residential areas with air strikes, long-range missiles and artillery.
The southern port of Mariupol has become a focal point of Russia's assault and is largely in ruins with bodies on the streets but attacks were also reported to have intensified on the second city of Kharkiv on Monday.
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.
Mr Biden said Russia's false accusations that Ukraine had biological and chemical weapons illustrated that President Vladimir Putin's "back was against the wall" and he was considering using such weapons.
"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 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."
Mr Biden also told businesses to be alert for cyber attacks by Russia. "It's part of Russia's playbook," he said in a statement.
The United States and its allies have previously accused Russia of spreading an unproven claim that Ukraine had a biological weapons programme as a possible prelude to using such weapons but Mr Biden's remarks on Monday were some of his strongest on the subject.
Russia says it does not attack civilians although the devastation wrought on Ukrainian towns such as Mariupol and Kharkiv are reminiscent of previous Russian assaults on cities in Chechnya and Syria.
Putin 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 that a false pretext for an unprovoked war of aggression.
No surrender
The conflict has driven almost a quarter of Ukraine's 44 million people from their homes.
Ukraine on Monday rejected a Russian demand to stop defending Mariupol.
The governor of the Zaporizhzhia region said buses evacuating civilians from front-line areas were hit by shelling on Monday and four children were wounded.
The eastern cities of Kharkiv, Sumy and Chernihiv have also been hard hit.
Among the dead in Kharkiv is Boris Romanchenko, a 96-year-old Holocaust survivor whose flat was shelled by Russian forces last week.
"Please think about how many things he has come through," Ukrainian President Volodymyr Zelenskiy said late on Monday.
"But (he) was killed by a Russian strike, which hit an ordinary Kharkiv multi-storey building. With each day of this war, it becomes more obvious what denazification means to them."
