The United States has announced sanctions on senior Russian government officials and Russian entities in response to what US officials said was Moscow's attempt to kill opposition leader Alexei Navalny with a nerve agent.
The announcement, made by senior Biden administration officials, marked a sharp turn away from former president Donald Trump's reluctance to confront Russian President Vladimir Putin.
Mr Navalny fell ill on a flight from Siberia in August and was airlifted to Germany, where doctors concluded he had been poisoned with a nerve agent.
The Kremlin has denied any role in his illness and said it had seen no proof he was poisoned.
The US officials said Mr Navalny had been targeted for his activism in trying to raise questions about what they called Russian corruption.
"The intelligence community assesses with high confidence that officers of Russia's Federal Security Service FSB used a nerve agent known as Novichok to poison Russian opposition leader Alexei Navalny on August 20, 2020."
US coordinates sanctions with EU
The officials said seven senior Russian government officials would face sanctions, such as asset freezes.
In addition, 14 entities associated with Russia's biological and chemical agent production, including 13 commercial parties and a government research institute, were levied punitive measures.
The targeted Russians will be restricted from travelling either to the European Union or the United States.
The US moves were being taken in coordination with the European Union.
The officials reiterated Mr Biden's call for Russia to release Mr Navalny from prison.
"We will take the appropriate actions as we see fit to make very clear that this kind of conduct is unacceptable for us, and we'll do it with our allies and partners," US Secretary of State Antony Blinken said.
Mr Biden is seen to be taking a tougher approach to Russia than Mr Trump but US officials said the sanctions were not an escalation.
"The United States is neither seeking to reset our relations with Russia, nor are we seeking to escalate," one official said.
"We believe that the United States and our partners must be clear and impose costs when Russia's behaviour crosses boundaries that are respected by responsible nations, and we believe there should be guard rails on how these adversarial aspects of our relationship play out."
Before the US announcement, Russian Foreign Minister Sergei Lavrov said Moscow would respond in kind to any new US sanctions over Mr Navalny, Russia's Interfax news agency reported.
The Kremlin has also denounced moves to impose sanctions.
"Those who continue to depend on these measures should probably give it some thought: are they achieving some goal by continuing such a policy?" Kremlin spokesman Dmitry Peskov said.
"The answer will be obvious — such a policy does not achieve its goals."
Further sanctions to be unveiled
After his medical treatment in Germany, Mr Navalny, 44, returned to Russia in January.
He was arrested and later sentenced to more than two-and-a-half years in jail for parole violations he said were trumped up.
Mr Navalny has persisted in needling Mr Putin, releasing a viral video that purported to show a palatial Black Sea residence belonging to the President, who was forced to deny publicly it was his.
The dissident has also mockingly said the veteran Russian leader "will go down in history as a poisoner of underpants" — referring to where agents allegedly placed the Novichok.
Further sanctions are upcoming, as the US assesses the Russian role in the massive SolarWinds cyber hack.
They are also looking into allegations Russia sought to interfere in the 2020 US election and offered bounties to Taliban fighters to kill American soldiers in Afghanistan, the officials said.