London, Nov 24 (IANS): Ruddian President Vladimir Putin could resort to using Novichok in mass chemical weapon attacks in Ukraine if his troops continue to lose ground, US officials fear, according to a media report.

As the progress of the war has gone increasingly in Kiev's favour, fears that Russia could take extreme measures to force Ukraine to surrender - such as launching a nuclear strike or a dirty bomb attack - have been rising, Daily Mail reported.

But according to six people with knowledge of the matter, Putin is prepared to use chemical weapons in a mass-casualty event before resorting to a nuclear confrontation with NATO.

According to the sources, cited by Politico, President Joe Biden's administration is working to ensure its western allies are prepared for such an attack.

The sources said that Washington predicts Russia will use chemical weapons in the event of further losses on the battlefield, or a total collapse of Putin's army.

Top US officials strategising for such an attack believe Moscow could deploy chemical weapons Russia has been known to use in the past - including those used in the poisoning of opposition leader Alexei Navalny (on a flight to Moscow) and former Russian military intelligence officer Sergei Skripal (in Salisbury, Britain), Daily Mail reported.

Both were poisoned with a Novichok nerve agent and were hospitalised in a critical condition. Against the odds, they both survived the attack.

While in the past, the nerve agent has been used by Russia to target individuals, US officials have said it can also be used to carry out a mass-casualty event.

Some chemicals can be turned into an aerosol or delivered using munitions to inflict devastating damage on a large area, and a large group of people.