Ukrainian President Volodymyr Zelenskyy says he fears Russia could use chemical weapons in Ukraine and urged the West to impose more severe sanctions against Russia to prevent it from using such weapons.
His comments Monday in his nightly video address came amid unconfirmed reports that chemical weapons have already been used in the besieged southern port city of Mariupol.
British Foreign Secretary Lizz Truss said Britain was aware of the reports.
"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," she said.
Pentagon press secretary John Kirby said the United States cannot at this time confirm the reports of chemical weapons use, but said if true, they "are deeply concerning."
He said the reports are "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."
Zelenskyy said a representative from a pro-Russia militia in Ukraine's eastern Donetsk region said Russia might use chemical weapons in Mariupol.
"We consider it very seriously," Zelenskyy said. He did not say if 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. And already at that time it meant that it was necessary to react to the Russian aggression much harsher and faster," Zelenskyy said.
Earlier Monday, Russia said it destroyed several air defense systems in Ukraine over the weekend, ahead of what Zelenskyy is warning could be the start of a renewed Russian offensive into the country's eastern region.
The Russian claims could not be verified, but Russian Defense Ministry spokesman Major General Igor Konashenkov said the military launched cruise missiles to destroy four launchers Sunday on the southern outskirts of the central city of Dnipro, and also hit systems in the Mykolaiv and Kharkiv regions.
Zelenskyy pleaded in a video address Monday for more military aid from the U.S. and its allies, specifically asking South Korean lawmakers to send more equipment that can shoot down Russian missiles.
He also said tens of thousands of people have been killed in Russia's assault on Mariupol.
Pentagon spokesperson Kirby said the United States was unable to confirm Ukrainian casualty figures in Mariupol because fighting there is ongoing but said the final death toll could be "a significant number."
The Pentagon said Monday it is seeing early signs of efforts by Russia to reinforce its troops in eastern Ukraine. The U.S. spotted a convoy north of Izyum with command-and-control elements, enablers, artillery and rotary blade air support, according to a senior U.S. defense official.
The official said there also are indications that Russian forces sent from northern Ukraine to Belarus and the Russian town of Valuyki are now moving toward eastern Ukraine.
The official said, "We do not assess a new offensive has started" in eastern Ukraine but added, "What is clear is that the Russians continue to sink to new lows of depravity and brutality as we saw with the missile strike on a train station last week and their continued assault on Mariupol.
"We're certainly bracing ourselves here for some potentially really, really horrible outcomes," the official said.
The U.S. official said Russia has launched 1,500 missile attacks on Ukraine during 47 days of war, destroying apartment buildings and hospitals throughout the country and killing thousands of Ukrainian civilians.
Russia has acknowledged sustaining "significant" troop losses of its own.
Russia said one of its latest missile attacks hit four S-300 launchers provided by a European country it didn't name. Slovakia gave Ukraine just such a system last week but denied it had been destroyed.
Military analysts say that Russia's failure to capture the Ukrainian capital of Kyiv and the bulk of territory throughout the country has in part been the result of its failure to control the skies over Ukraine to provide cover for ground troops. Nonetheless, its missile bombardment has virtually flattened some cities, and one missile attack on a train station killed more than 50 people last Friday.
Russia has not been able to stop the flow of more military aid to Ukraine, with eight to 10 flights arriving daily.
Pentagon spokesperson Kirby said that U.S. security assistance is getting to Ukraine in as little as four to six days.
"That's an incredible rate of speed, and we aren't slowing down," he said.
Zelenskyy told the CBS News show "60 Minutes" on Sunday that Ukraine's fate depends on further Western military assistance.
"To be honest, whether we will be able to (survive) depends on this," Zelenskyy said. "Unfortunately, I don't have the confidence that we will be receiving everything we need."
Meanwhile, Austrian Chancellor Karl Nehammer met with Russian President Vladimir Putin in Moscow, the first such visit by a European leader since Putin launched the invasion of Ukraine on February 24.
In a statement after the meeting, Nehammer said the discussion with Putin was "very direct, open and tough." Nehammer said his most important message to the Russian leader was that the war in Ukraine must end because "in a war there are only losers on both sides."
There was no immediate comment from the Kremlin.
VOA's Ken Bredemeier and Jeff Seldin contributed to this report. Some information in this report came from Reuters and The Associated Press.