The US has "full confidence" in Britain's assessment that Russia was "likely responsible" for a nerve agent attack on a former Russian spy in Britain last week, Secretary of State Rex Tillerson says.

"There is never a justification for this type of attack - the attempted murder of a private citizen on the soil of a sovereign nation - and we are outraged that Russia appears to have again engaged in such behaviour," Tillerson said.

The top US diplomat said he had spoken to British Foreign Secretary Boris Johnson and that the US stood "in solidarity with our allies in the United Kingdom" and would "continue to co-ordinate closely our responses".

He added: "We agree that those responsible - both those who committed the crime and those who ordered it - must face appropriately serious consequences".

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The UK says former Russian spy Sergei Skripal and his daughter Yulia were poisoned with a "military grade" nerve agent produced by Russia.

British Prime Minister Theresa May told Parliament it is "highly likely" that Russia was behind the attack.

- DPA, AAP