Washington said on Wednesday it would impose fresh sanctions on Russia after it determined that Moscow had used a nerve agent against a former Russian agent and his daughter in Britain.

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 the southern English city of Salisbury in March after a liquid form of the Novichok type of nerve agent was applied to his home's front door.

State Department spokeswoman Heather Nauert said 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 Aug. 22.

The news came as Republican US Senator Rand Paul said on Wednesday he had delivered a letter from President Donald Trump to Russian President Vladimir Putin proposing cooperation.

The sanctions would be structured in two tranches, with the biggest impact from the initial sanctions expected to come from a ban on granting licenses to export sensitive national security goods to Russia, NBC reported, quoting a senior State Department official.

British police have identified several Russians who they believe were behind the nerve agent attack on former spy Sergei Skripal and his daughter.

Britain blamed Russia for the poisonings and identified the poison as Novichok, a deadly group of nerve agents developed by the Soviet military in the 1970s and 1980s. Russia has repeatedly denied any involvement in the attack.