The US will impose fresh sanctions on Russia after it determined that Moscow had used a nerve agent against a former Russian spy and his daughter in Britain.

A senior State Department official on Wednesday said it had notified the Kremlin of the sanctions.

Sergei Skripal, a former colonel in Russia’s GRU military intelligence service, and his 33-year-old daughter, Yulia, were found unconscious 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.

US 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.”

The sanctions will cover sensitive national security goods, a senior State Department official told reporters, citing the 1991 Chemical and Biological Weapons and Warfare Elimination Act.

There would, however, be exemptions for space flight activities and areas covering commercial passenger aviation safety, which would be allowed on a case-by-case basis.

The official said a second batch of “more draconian” sanctions would be imposed after 90 days unless Russia gives “reliable assurances” that it will no longer use chemical weapons and allow on-site inspections by the United Nations.

“If those criteria are not met – it is up to Russia to make that decision – a second round of sanctions … will to be imposed,” the official said,

“They are in general more draconian than the first round.”

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.

-AAP