Russia will decide on retaliatory measures to US sanctions imposed over a nerve agent attack in Britain blamed on Moscow, which it denies, the foreign ministry said on Thursday.

"The Russian side will work on developing retaliatory measures," ministry spokeswoman Maria Zakharova told journalists.

The Kremlin earlier Thursday called the latest action by the US State Department "unacceptable." 

The announcement of US sanctions caused the value of the ruble to plunge early Thursday and Russian stock markets to fall.

Zakharova said the United States was "knowingly presenting demands that are unacceptable to us" as conditions for the sanctions to be lifted.

"We are being threatened with further escalation of sanctions pressure," she said.

"In this way, the US is consciously taking the path of further heightening of tensions in bilateral relations that have already been brought practically to zero by their efforts."

She accused Washington of picking the nerve agent poisoning of former Russian double agent Sergei Skripal and his daughter in Salisbury, England, as a "contrived excuse" for sanctions.

Russia has denied any involvement in the Skripals' poisoning and also the subsequent death of a British woman Dawn Sturgess after she was exposed to the agent.