The US and UK governments are voicing concerns Russia may deploy biological and chemical weapons during its assault on Ukraine, warning Moscow may deploy "false flag" tactics to justify such attacks.
Top European Union officials, meanwhile, have condemned Russia's bombing of a maternity hospital in the besieged Ukrainian city of Mariupol as a "war crime", but Russia maintains the air strike did not target civilians.
The US slammed Russia and China for spreading "conspiracy theories" that the US was operating biological and chemical weapons labs near the Ukraine-Russia border.
White House Press Secretary Jen Psaki called the claims "preposterous" and said it was "the kind of disinformation operation we've seen repeatedly from the Russians … and an example of the types of false pretexts we have been warning the Russians would invent".
"It's Russia that has a long and well-documented track record of using chemical weapons, including in attempted assassinations and poisoning of Putin's political enemies like Alexei Navalny," Ms Psaki said.
A military false flag operation is one conducted with the intent of disguising the source and apportioning blame to another party.
Russia has used the nerve agent Novichok to target Mr Putin's critics and has been accused of staging chemical weapons attacks during the war in Syria.
Ukraine President Volodymyr Zelenskyy said in a video address in the early hours of Friday local time, that he was worried Russia would use chemical weapons against his country.
Mr Zelenskyy said Russia's accusations of Ukraine developing biological weapons was a sign they were intending to use that weaponry in this invasion.
"They accuse us, again us, that we are allegedly developing biological weapons. That we are preparing a chemical attack," he said.
Asked whether the use of chemical weapons would be a "red line" that would trigger the deployment of US troops to Ukraine, Ms Psaki said: "We don't have any intention to do that."
US State Department spokesman Ned Price said Russia was "inventing false pretexts in an attempt to justify its own horrific actions in Ukraine".
British Prime Minister Boris Johnson echoed the concerns, saying he feared Russia would deploy chemical weapons in Ukraine.
"The stuff that you're hearing about chemical weapons, this is straight out of their playbook," Mr Johnson said in an interview with Sky News.
"They start saying that there are chemical weapons that have been stored by their opponents or by the Americans.
Earlier, Russian foreign ministry spokeswoman Maria Zakharova said Russia had documents showing the Ukrainian health ministry had ordered the destruction of samples of plague, cholera, anthrax and other pathogens before February 24, when Russian forces moved into Ukraine.
Ms Zakharova said the documents unearthed by Russian forces in Ukraine showed "an emergency attempt to erase evidence of military biological programs" financed by the Pentagon.
She provided no further details on the documents.
A Ukrainian presidential spokesperson said: "Ukraine strictly denies any such allegation."
Like many other countries, Ukraine has public health laboratories researching how to mitigate the threats of dangerous diseases affecting both animals and humans.
Its laboratories have received support from the US, European Union and the World Health Organization.
Mariupol hospital strike 'inhumane, cruel'
European Commission head Ursula von der Leyen and foreign policy chief Joseph Borrell expressed shock over Wednesday's attack on the hospital, which Ukrainian President Volodymyr Zelenskyy described as a Russian "war crime".
"Mariupol is under siege. Russia's shelling of [a] maternity hospital is a heinous war crime," Mr Borrell wrote on Twitter.
"Strikes of residential areas from the air and blocks of access of aid convoys by the Russian forces must immediately stop. Safe passage is needed, now," he added.
Ms von der Leyen called the attack "inhumane, cruel and tragic".
Local officials said the air strike killed three people and left 17 people injured.
Russia's defence ministry denied bombing the maternity and children's hospital in Mariupol and accused Ukraine of a "staged provocation".
The comments came after Russia and Ukraine's foreign ministers failed to agree on any measures to ease a conflict that has trapped hundreds of thousands of civilians in Ukrainian cities sheltering from Russian air raids and shelling.
Thousands of people have been killed and more than 2 million refugees have fled Ukraine since Russian troops invaded on February 24.
Local authorities in Mariupol, meanwhile, were hurrying to bury the dead in a mass grave.
City workers made quick sign-of-the-cross gestures as they pushed bodies wrapped in carpets or bags into a deep trench some 25 metres' long on the outskirts of the city.
More than 70 bodies have been interred in the grave since it was opened on Tuesday.
'They want to delete our city'
Mariupol officials said Russian warplanes were again relentlessly bombing the city, which had been under siege for the past 10 days.
Aid agencies say humanitarian help is urgently needed in the city, where residents have been trapped with no food, water or power.
Attempts to send aid and evacuation convoys have failed for six days after several ceasefire agreements failed.
Russian aircraft were targeting convoy routes, Petro Andrushenko, an adviser to Mariupol's Mayor, said
"We try and try and try, but I'm not sure if it'll be possible today — or other days," Mr Andrushenko said.
"Air strikes started from the early morning. Air strike after air strike. All the historic centre is under bombardment," he said.
There were reports of Russian forces blocking civilians from leaving other cities, including the eastern Ukrainian town of Izyum.
Regional Governor Oleh Synegubov said Ukrainian authorities were only able to help around 1,600 people flee the part of Izyum held by Ukraine, he said in televised comments.
Putin increasingly isolated as Russia retaliates against Western sanctions
Russian President Vladimir Putin remained defiant amid international condemnation of the hospital strike, declaring Russia would ultimately emerge stronger after overcoming the difficulties caused by economic sanctions.
He told a government meeting there had been no alternative to what Russia calls its special military operation in Ukraine.
"There are some questions, problems and difficulties, but in the past we have overcome them and we will overcome them," he said.
Mr Putin also said Moscow — which supplies a third of Europe's gas — would continue to meet its contractual obligations.
Speaking calmly, the Kremlin leader acknowledged that sanctions imposed since February 24 were being felt.
"It is clear that at such moments people's demand for certain groups of goods always increases, but we have no doubt that we will solve all these problems while working in a calm fashion," he said.
"Gradually, people will orient themselves, they will understand that there are simply no events that we cannot close off and solve."
Russia has sought to retaliate against Western sanctions imposed over its invasion of Ukraine by banning exports of certain goods and agricultural commodities.
Exporting telecom, medical, auto, agricultural, electrical and tech equipment, as well as some forestry products, will be banned until the end of 2022.
"These measures are a logical response to those imposed against Russia and are aimed at ensuring uninterrupted functioning of key sectors of the economy," the economy ministry said.
Further measures could include restricting foreign ships from entering Russian ports and allowing Russian airlines to register jets leased from Western firms as their own property, the government said.
Western companies have pulled out of Russia en masse as the US, EU and UK have imposed sanctions aimed at curbing Moscow's access to funding.
The British government on Thursday blacklisted more Russian oligarchs, including Britain's best known: Roman Abramovich, owner of the Chelsea football team.
ABC/wires
