Zelensky offers to swap pro-Putin tycoon for captured Ukrainians

In the latest discovery fuelling allegations of Russian atrocities, Ukrainian prosecutors said six people had been found shot dead in the basement of a building outside the capital. 

While the toll on towns occupied during the month-long offensive to take Kyiv is still coming to light, the heaviest civilian toll is feared to be in Mariupol, where Zelensky said he believed Russia had killed "tens of thousands."

Journalists in Mariupol, as part of a Russian military embed, witnessed the charred remains of the city, including the theatre where 300 people were feared killed in Russian bombardment last month.

As fighting dragged toward its seventh week, the Ukrainian army was fighting desperately to defend strategically located Mariupol.

Moscow is believed to be trying to connect occupied Crimea with Russian-backed separatist territories Donetsk and Lugansk in Donbas, and has laid siege to the city, once home to more than 400,000 people.

Reports emerged on Monday from Ukraine's Azov battalion that a Russian drone had dropped a "poisonous substance" in the area, with people experiencing respiratory failure and neurological problems.

US Secretary of State Antony Blinken said he was unable to confirm the allegations, but that Washington had "credible information" Russia might use tear gas mixed with chemical agents in the besieged port.

The world's chemical weapons watchdog said it was "concerned" by the unconfirmed reports coming from Mariupol, and was "monitoring closely."

Pentagon spokesman John Kirby warned the use of such weapons by Moscow would "elicit a response not just from the United States, but from the international community," without elaborating.

Ukrainian President Volodymyr Zelensky offered Wednesday to swap pro-Kremlin tycoon Viktor Medvedchuk, who was arrested by Kyiv after escaping house arrest, for Ukrainians captured by Russia.

Medvedchuk, 67, counts Russian President Vladimir Putin among his personal friends and says the Kremlin leader is godfather to his youngest daughter Darya.

"I propose to the Russian Federation to exchange this guy of yours for our boys and our girls who are now in Russian captivity," Zelensky said in a video address posted on Telegram.

"And may Medvedchuk be an example for you. Even the former oligarch did not escape. What can we say about much simpler criminals from the Russian hinterland? We will get everyone."

Ukrainian authorities announced Tuesday they had captured a prominent pro-Kremlin tycoon who escaped from house arrest after Russia's invasion.