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Ukrainian President Volodymyr Zelenskyy said on Monday that Russia could employ chemical weapons in Ukraine and called on the West to impose strong sanctions that would deter even talk of using such arms.
There were unconfirmed reports on Monday suggesting that chemical weapons were used in the besieged southern Ukrainian port of Mariupol.
"We treat this with the utmost seriousness," Mr Zelenskyy said late on Monday. He did not say chemical weapons had already been used.
"I would like to remind world leaders that the possible use of chemical weapons by the Russian military has already been discussed," he said.
"And already at that time it meant that it was necessary to react to the Russian aggression much harsher and faster."
Petro Andryushchenko, an aide to the mayor of Mariupol, wrote on his Telegram channel that reports of a chemical attack had not been confirmed and that he expected to provide details and clarification later.
The UK Foreign Secretary Liz Truss took to Twitter over the claims.
"Reports that Russian forces may have used chemical agents in an attack on the people of Mariupol," Ms Truss said. "We are working urgently with partners to verify details.
"Any use of such weapons would be a callous escalation in this conflict and we will hold [Russian President Vladimir] Putin and his regime to account."
In the US, Pentagon press secretary John Kirby said the department was aware of the reports.
"We cannot confirm at this time and will continue to monitor the situation closely," Mr Kirby said.
"These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia's potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine."
Ukrainian politician Ivanna Klympush-Tsintsadze said Russia had used an "unknown substance" in Mariupol and that people were suffering from respiratory failure.
"Most likely chem weapons," Ms Klympush-Tsintsadze tweeted.
Ukraine's Azov battalion in a Telegram message earlier on Monday claimed a Russian drone dropped a "poisonous substance" on Ukrainian troops and civilians in Mariupol.
It also claimed that people were experiencing respiratory failure and neurological problems.
The battalion's founder, Andriy Biletsky, said that three people were suffering effects from an unknown toxic substance.
"Three people have clear signs of poisoning by warfare chemicals, but without catastrophic consequences," he said in a video address on Telegram.
AFP was unable to verify the claims.
Senior Donetsk separatist official Eduard Basurin had spoken of the possibility of using chemical weapons against the southern port city, which has resisted Russian bombardment for weeks.
Mr Basurin said the besieging forces could "turn to chemical troops who will find a way to smoke the moles out of their holes", Russian news agency RIA Novosti quoted him as saying on Monday.
Russia has denied committing any war crimes during its offensive in Ukraine.
The EU said on Monday that more sanctions on Russia were an option.
"It is time to make this package in such a way that we would not hear even words about weapons of mass destruction from the Russian side," Mr Zelenskyy said.
"An oil embargo against Russia is a must. Any new package of sanctions against Russia that does not affect oil will be received in Moscow with a smile."