Russian president Vladimir Putin could employ chemical or biological weapons against Ukrainian forces defending against his invasion of their country, US officials have said.

       On Sunday, Mr Putin ordered Russian nuclear forces to an increased alert level, citing what he called “aggressive statements” from Nato member countries and the harsh economic sanctions levelled upon Russia in response to the invasion he ordered last week.

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       But Russia also possesses stockpiles of biological and chemical weapons in addition to the nuclear arsenal it inherited from the Soviet Union, raising the possibility that Mr Putin could order their use if he feels his troops are faltering.

       Speaking on CBS News programme Face the Nation, US ambassador to the United Nations Linda Thomas-Greenfield said Mr Putin is capable of using the weapons of mass destruction if it furthers his goal of taking over Ukraine.

       “Certainly nothing is off the table with this guy. He's willing to use whatever tools he can to intimidate Ukrainians and the world,” she said.

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       Ms Thomas-Greenfield is the second US official in recent weeks to raise the possibility that Mr Putin could choose to employ weapons of mass destruction against Ukraine.

       During an 18 February meeting of the UN Security Council, secretary of state Anthony Blinken warned that “conventional attacks” were “not all that Russia plans to inflict upon the people of Ukraine”.

       While Russia is a signatory to international conventions banning the use of chemical weapons, another Russian ally – Syrian dictator Bashar al-Assad – has used chemical weapons in the long-running civil war in his country.

       US Ambassador to the UN Linda Thomas-Greenfield says Putin could use dirty weapons in Ukraine

       (CBS/Face The Nation)

       Russian security services have also employed a Russian-made nerve agent, novichok, against defectors living outside of Russia before.

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       One such defector, Sergei Skripal, was poisoned with novichok along with his daughter Yulia while living in Salisbury, Wiltshire in March 2018.

       Three other people in the UK were also hospitalised after exposure remnants of the substance, one of whom later died.