British and US officials are warning the world to be "on the lookout" for a chemical or biological weapon attack by Russia in Ukraine.
The Kremlin has accused the US of supporting a bioweapons programme in Ukraine, without any firm evidence.
The White House rubbished that claim, calling it an "obvious ploy" to try justify an unprovoked attack.
Earlier this week, the British government said Russia had launched thermobaric rockets - or vacuum bombs - on Ukraine.
Andrew Weber is a former assistant secretary of defence for the Obama administration, and now sits on the Council on Strategic Risks.
He spoke to Corin Dann.
