The United States has said it is to impose new sanctions on Russia, as it accused Moscow of using a "lethal" nerve agent in the attempted assassination of a former spy in Britain.

The State Department said the sanctions were in response to "the use of a 'Novichok' nerve agent in an attempt to assassinate UK citizen Sergei Skripal" - who was a double agent - and his daughter Yulia in March.

The action is aimed at punishing Russian President Vladimir Putin's government for having "used chemical or biological weapons in violation of international law," State Department spokeswoman Heather Nauert said in a statement.

The new sanctions, details of which were not immediately released, were to take effect following a 15-day Congressional notification period, she said.

The action follows the US Treasury's imposition of sanctions in March against 19 Russian citizens and five entities for interfering in the 2016 US election - the toughest steps against Moscow since US President Donald Trump took office.

They came after the US intelligence community's conclusion that Russia tried to help Mr Trump win the 2016 presidential race.

The new announcement could bolster President Trump's claim that his administration is taking a tough stance on Moscow, even as he denounces special counsel's Robert Mueller's Russia probe as a "witch hunt" that should be halted immediately.

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Mr Trump was criticised by Democrats and Republicans alike for what many saw as his unsettling embrace of Mr Putin last month at their Helsinki summit, when he appeared to disavow his own intelligence agencies' assessment on Russia's election interference.

Sergei and Yulia Skripal were found unconscious on a bench in Salisbury, England, on 4 March, having been poisoned by Novichok, a military-grade nerve agent developed by the Soviet Union during the Cold War.

They were critically ill, but survived after spending weeks in the hospital.

Russia has strongly denied playing a role in the attack.

On 30 June, a British couple was poisoned by Novichok in a nearby town - 44-year-old mother-of-three Dawn Sturgess subsequently died.

The UK and its allies have accused Russia of trying to kill the Skripals and say the two cases could be linked.

The British government welcomed the US decision to impose fresh sanctions on Russia.

"The UK welcomes this further action by our US allies," a spokesman for the UK Foreign Office said in a statement.  

"The strong international response to the use of a chemical weapon on the streets of Salisbury sends an unequivocal message to Russia that its provocative, reckless behaviour will not go unchallenged."