"The stuff that you're hearing about chemical weapons, this is straight out of their playbook," Johnson said in an interview with Sky News.

"They start saying that there are chemical weapons that have been stored by their opponents or by the Americans, and so when they themselves deploy chemical weapons, as I fear they may, they have a sort of ... fake story, ready to go."

The comments on Thursday come a day after the US denied renewed Russian accusations that Washington was operating biowarfare labs in Ukraine.

It called the claims "laughable" and suggested Moscow may be laying the groundwork to use a chemical or biological weapon.

Earlier British Foreign Secretary Liz Truss warned it would be a grave mistake for Russian President Vladimir Putin to use chemical weapons.

"We have seen Russia use these weapons before in fields of conflict, but that would be a grave mistake on the part of Russia, adding to the grave mistakes already being made by Putin," she told CNN.

Also on Thursday Britain's armed forces minister James Heappey was asked whether the use of chemical weapons might cross a "red line" that could cause NATO members to get involved in the conflict.

"I don't think it is 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," he told BBC radio.

"I think he (Putin) should reflect very urgently on what has happened to other countries where they have used this."

Australian Associated Press