Defence minister James Heappey has warned Russia that the use of chemical weapons has "triggered an international response" in the past.
Western officials are concerned that Vladimir Putin could mount such an attack as the Russian tactics become increasingly brutal.
Mr Heappey told BBC Radio 4's Today programme: "I don't think it's helpful to get into any firm commitment right now about where that red line sits, but I think President Putin needs to be very clear that when other countries have used chemical weapons it has caused an international response.
