The US Government says it will impose fresh sanctions on Russia after it determined that Moscow used a nerve agent against a former Russian agent and his daughter in the UK.

Sergei Skripal, a former colonel in Russia's GRU military intelligence service, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench in March after a liquid type of nerve agent was applied to his home's front door.

State Department spokeswoman Heather Nauert said on Wednesday it had been determined that Russia "has used chemical or biological weapons in violation of international law, or has used lethal chemical or biological weapons against its own nationals".

She said sanctions would take effect on or around August 22nd.