“Mariupol has been destroyed, there are tens of thousands of dead, but even despite this, the Russians are not stopping their offensive,” President Volodymyr Zelenskiy said in a video address to South Korean lawmakers.

Reuters has confirmed widespread destruction in Mariupol but could not verify the accuracy of his estimate of those killed in the strategic city, which lies between Russian-annexed 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 many bodies, including of civilians, have been seen in the streets.

The head of the Russia-backed self-proclaimed Donetsk People’s Republic, 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.

Petro Andryushchenko, an aide to the mayor of Mariupol, said on the Telegram messaging service that numbers of people leaving the city had fallen, not because people did not want to escape but because Russian forces had slowed pre-departure checks.

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.

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, Ukraine’s Deputy Prime Minister Iryna 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. Moscow calls its invasion of Ukraine a “special military operation”.