There are calls from the US and Britain for an investigation after Ukrainian soldiers of the Azov regiment were reportedly injured by a "poisonous substance" during a Russian attack on the port city of Mariupol. No evidence has yet been gathered. The UK foreign secretary says it would be "a callous escalation" of the war. The Pentagon says it's "deeply concerning". Ukraine's President Volodymyr Zelensky has already expressed his fears that Russia could deploy chemical weapons in his country.