The US is to impose sanctions on Russia after determining that it used nerve agent against former Russian double agent Sergei Skripal in the UK in March.
The move was announced on Wednesday by the US state department.
A UK investigation blamed Russia for the attack, but the Kremlin has strongly denied any involvement.
Mr Skripal and his daughter Yulia were found unconscious on a bench in the city of Salisbury on 4 March.
They were seriously ill but recovered after spending several weeks in hospital.
The US determined “that the government of the Russian Federation has used chemical or biological weapons in violation of international law, or has used lethal chemical or biological weapons against its own nationals”, US state department spokeswoman Heather Nauert said.
The sanctions are to take effect on or around 22 August, she added.
- What happened to the Skripals?
- Will Russian suspects face UK trial?
- Who is Sergei Skripal?
- What are nerve agents and what do they do?
The British government welcomed the move.
“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,” a Foreign Office statement said.
What are the sanctions?
The new sanctions include exports of sensitive electronic components and other technologies.
The state department said further sanctions would follow within 90 days if Russia did not give assurances that it had stopped the use of chemical weapons.
An official said it was only the third time that the US had determined a country had used chemical or biological weapons against its own nationals.
Previous occasions were against Syria and against North Korea for the assassination of Kim Jong-nam, the half brother of leader Kim Jong-un, who died when highly toxic VX nerve agent was rubbed on his face at Kuala Lumpur airport.
Are these the only US sanctions against Russia?
No. In June the US slapped sanctions on five Russian companies and three Russian individuals in response to alleged Russian cyber-attacks on the US.
All are prohibited from any transactions involving the US financial system.
Treasury Secretary Steven Mnuchin said the measures were to counter “malicious actors” working to “increase Russia’s offensive cyber-capabilities”.
Russia likely to resist
Analysis by Gary O’Donoghue, BBC News, Washington
After pressure from Republican members of Congress, the state department has determined Moscow broke international law by using a military grade chemical weapon on the Skripals.
While the US expelled some five dozen diplomats shortly after the poisoning, the administration stopped short of making a formal determination that Russia had broken international law.
But Congress has been pushing for such a decision and now the state department has confirmed Russia’s actions contravened 1991 US legislation on the use of chemical weapons. That breach automatically triggers the imposition of sanctions and places requirements on Russia to avert further restrictions in three months time.
Those requirements could include opening up sites in Russia for inspection – a move Moscow would probably resist.
So far President Donald Trump has been silent on this latest move – which could well derail his attempts to develop a new, warmer relationship with Vladimir Putin.
What was the nerve agent?
Following the incident, the British government said the military-grade nerve agent Novichok, of a type developed by Russia, had been used in the attack.
Relations between Russia and the West hit a new low. More than 20 countries expelled Russian envoys in solidarity with the UK, including the US. Washington ordered 60 diplomats to leave and closed the Russian consulate general in Seattle.
Three months after the Salisbury attack, two other people fell ill at a house in Amesbury, about eight miles from the city. Dawn Sturgess later died while her partner, Charlie Rowley, spent three weeks recovering in hospital.
After tests, scientists at the UK’s military research lab, Porton Down, found the couple had also been been exposed to Novichok.
Mr Rowley told ITV News he had earlier found a sealed bottle of perfume and given it to Ms Sturgess, who sprayed the substance on her wrists.
Skripal attack: US to sanction Russia over Novichok use