KYIV (REUTERS, AFP) - Ukraine on Monday (April 11) said tens of thousands of people have likely been killed in Russia's assault on the southeastern city of Mariupol while the country's rights ombudsman accused Russian forces in the region of torture and executions.

There were also unconfirmed reports on Monday suggesting chemical weapons were used in Mariupol.

Ukrainian President Volodymyr Zelensky said Russia could use chemical weapons in his country and called on the West to impose strong sanctions on Moscow that would deter even talk of the use of such weapons.

"We treat this with the utmost seriousness," Mr Zelensky 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. And already at that time it meant that it was necessary to react to the Russian aggression much harsher and faster."

Britain is trying to verify reports that Russia has used chemical weapons in Mariupol, London’s top diplomat said on Monday.

“Reports that Russian forces may have used chemical agents in an attack on the people of Mariupol. We are working urgently with partners to verify details,” Foreign Secretary Liz Truss wrote on Twitter.

“Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account.”

Ukrainian lawmaker Ivanna Klympush said Russia had used an “unknown substance” in Mariupol and that people were suffering from respiratory failure. “Most likely chem.weapons!” she posted on Twitter.Ukraine’s Azov battalion in a Telegram message earlier on Monday had claimed a Russian drone dropped a “poisonous substance” on Ukrainian troops and civilians in Mariupol.

The force also claimed that people were experiencing respiratory failure and neurological problems.

The battalion’s founder Andrei 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 that 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.

Meanwhile, Reuters has confirmed widespread destruction in Mariupol but could not verify the alleged crimes or the estimate of those killed in the strategic city, which lies between Russian-occupied Crimea and eastern areas of Ukraine held by Russian-backed separatists.

If confirmed, it would be by far the largest number of dead so far reported in one place in Ukraine, where cities, towns and villages have come under relentless bombardment and bodies, including civilians, have been seen in the streets.

The head of the Russia-backed self-proclaimed Donetsk People's Republic, Mr Denis Pushilin, told Russia's RIA news agency on Monday that more than 5,000 people may have been killed in Mariupol. He said Ukrainian forces were responsible.

The numbers of people leaving the city had fallen because Russian forces had slowed pre-departure checks, Mr Petro Andryushchenko, an aide to the mayor of Mariupol, said on Monday on the Telegram messaging service.

Around 10,000 people were awaiting screening by Russian forces, he said. Russia does not allow military personnel to leave with civilian evacuees. There was no immediate comment from Moscow, which has previously blamed Ukraine for blocking evacuations.

Citing figures from Mariupol's city administration, Ukraine's human rights ombudsman Lyudmyla Denisova said 33,000 residents of Mariupol had been deported to Russia or territories held by Russian-backed separatists in eastern Ukraine. Russia said on Sunday it had "evacuated" 723,000 people from Ukraine since the start of what it called its "special operation". Moscow denies attacking civilians.

"Witnesses report that Russian national guard troops and 'Kadyrovite' (Chechen) units are making illegal arrests, torturing detainees and executing them for any pro-Ukrainian stance," in Mariupol, Ms Denisova said in a post on Telegram.

The Russian government did not immediately respond to a request for comment on the torture allegations.

Ukrainian Interior Ministry Adviser Anton Geraschenko said in a televised interview on Monday that Ukrainian "deportees"were kept in guarded sanatoria and holiday camps.

"These people are not allowed to move around freely, or to have free access to communication platforms in order to contact their relatives in Ukraine," he said, without citing direct evidence.

Ukraine's Deputy Prime Minister Iryna Vereshchuk told Reuters that the number of checkpoints along the Russian-controlled corridor between Mariupol to the Ukrainian city of Zaporizhzhia had grown from three to 15.

Mariupol was among nine humanitarian corridors agreed with Russia on Monday to evacuate people from besieged eastern regions, but its corridor was for private cars only, Ms Vereshchuk said on Telegram.

It was not possible to agree the provision of buses, she said.

Ukraine says Russian forces are massing for a new offensive on eastern areas, including Mariupol, where people have been without water, food and energy supplies for weeks.