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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.
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A young woman looks out the window as her train arrives from Pokrovsk, in the east of Ukraine, to Lviv in the country's centre. Getty -
A truck carries black bags containing bodies exhumed from a mass grave for investigations in Bucha, in the outskirts of Kyiv, Ukraine. AP -
A child uses an air rifle to shoot at a target with the face of Russian President Vladimir Putin in Lviv, Ukraine. Getty -
Firefighters try to contain a blaze at a factory after Russian shelling in Kharkiv, Ukraine. Reuters -
A woman reads as residents shelter from Russian shelling in a Kharkiv metro station. Reuters -
Ukrainian rescuers try to put out the fire at a private building after shelling in Kharkiv, Ukraine. EPA -
A couple of residents walk past burning shops after a Russian attack in Kharkiv, Ukraine. AP Photo -
A destroyed Russian tank on the outskirts of Buzova village in Kyiv draws crowds of curious residents. AFP -
The Russian offensive in the Kyiv region has left a huge crater on a bridge in Makariv. AFP -
Workers put an Ukrainian flag on a pole in the centre of Makariv. AFP -
A Ukrainian mother is comforted by relatives after the body of her son was discovered in a manhole in Buzova village, west of Kyiv. AFP -
An injured Ukrainian on a medical evacuation train bound for the western Ukrainian city of Lviv. AFP -
A Doctors Without Borders team at work on medical evacuation train. AFP -
A damaged building in the southern port city of Mariupol. Reuters -
Graves of the victims of the Ukraine-Russia conflict in Mariupol. Reuters -
Emergency workers sift through debris of a damaged building in Mariupol. Reuters -
The conflict has forced Mariupol residents to wait in line for drinking water. Reuters -
A Russian military vehicle in Mariupol. Reuters -
Food distribution at a church in the town of Borodyanka, north-west of Kyiv. AP -
A visitor is in shock after returning to her church, which had come under Russian attack, in the Kyiv town of Makaro. AP -
Mourners gather at the funeral of Ukrainian soldier Andriy Zagornyakon in Kamianka-Buzka. Getty Images -
A resident returns with his dog to survey his damaged building in Kharkiv. EPA -
A woman and a child are forced to sit on the corridor of a building in Kharkiv that is used as a bomb shelter. EPA -
A Ukrainian guard in the eastern town of Barvinkove, during Russia's invasion of its neighbouring country. AFP -
Ukrainian guards strengthen their position in the eastern town of Barvinkove. AFP -
Emotional farewell at the train station in the eastern Ukrainian city Kramatorsk, which has come under Russian attack. AFP -
A Ukrainian serviceman surveys the damaged cultural centre, in the town of Rubizhne, Luhansk region. AFP -
A participant at the pro-Ukrainian demonstration outside Downing Street in London. Reuters -
Colleagues bid farewell to Yuriy Dadak-Ruf and Taras Kryt, the Ukrainian soldiers killed in Russian shelling of the Luhansk region. Reuters -
A villager warms himself in the yard of his house in Bucha, on the outskirts of Kyiv. AP -
Vlad, the six-year old Ukrainian who lost his mother during their confinement in a Bucha basement amid the Russian invasion, plays with a friend inside his house. AP -
A group of women wait to receive free food in Bucha. AP -
Shakhtar Donetsk players carry the Ukrainian flag before their football match against Olympiakos in Piraeus, near Athens. AP -
Rescue workers clear the rubble of an apartment building in Borodianka. Getty Images -
A man repairs the roof of a house while standing on a ladder, in Andriivka, Ukraine. Getty Images -
Valerii, 65, rides by his destroyed apartment building in Borodianka. Getty Images -
A Ukrainian woman in Borodianka waits as rescue workers search for her daughter, her son-in-law and her granddaughter among the rubble. Getty Images -
Family photos are scattered among the debris of a destroyed apartment tower in Borodianka. Getty Images -
A distraught survivor of the attack on Borodianka. Getty Images -
Debris of an apartment building in Borodianka. Getty Images -
A writing on the gate of a destroyed building in Andriivka - 'Shells, mines?' - points to the threat faced by Ukrainians. Getty Images -
Another writing on the fence of a destroyed building, in Andriivka, reads 'People children live here'. Getty Images -
A bullet-riddled car near Ozera. Getty Images
"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."
