The United States on Tuesday imposed sanctions on seven senior Russians as it said its intelligence concluded that Moscow was behind the poisoning of jailed Kremlin critic Alexei Navalny.

In action coordinated with the EU, the United States renewed demands that Russia free Navalny, who was arrested in January upon his return to Moscow as he spurred massive rallies through his allegations of corruption by President Vladimir Putin.

"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," a senior US official said.

Officials said that the United States would impose sanctions on "seven senior members of the Russian government" with the details expected to be released later Tuesday. 

They also said that the United States would restrict exports to Russia as it vowed that President Joe Biden would take a harder line than his predecessor Donald Trump, who voiced admiration for Putin.

"We're sending a clear signal to Russia that there are clear consequences to the use of chemical weapons," another official said.

Navalny, 44, fell violently ill when he was on a domestic flight. 

He was rushed to treatment in Germany where doctors said he had been poisoned with Novichok, a nerve agent developed by Soviet researchers and which was also blamed in a 2018 attack in England against Russian double-agent Sergei Skripal and his daughter and Yulia.