Ukraine invasion: Expert on Russia's chemical weapons

In Ukraine, a defiant President Volodymyr Zelensky says his country is on the brink of surviving the Russian invasion, almost four weeks after it began.
His optimism, in an address to the Italian parliament, comes despite a warning from the Ukrainian military that Russia will continue to indiscriminately shell cities and infrastructure across the country.
Intense fighting is still centred around Mariupol, where there are reports some Russian forces are now inside the beseiged southern port city.
Ninety percent of buildings in the city are reportedly damaged or destroyed, while about 300,000 citizens there must contend with conditions described as "a freezing hellscape".
US president Joe Biden has said there are clear signs that Russian counterpart Vladimir Putin is becoming impatient and may be preparing to use chemical and biological weapons.
BBC security correspondent Frank Gardner spoke to Corin Dann.