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 on Thursday called the latest action by the US State Department "illegal" and "unacceptable."
"Making a linking to these events (the British poisoning) is for us unacceptable and such restrictions like those passed by the American side earlier … are absolutely illegal and do not correspond to international law," said Kremlin spokesman Dmitry Peskov, according to a Reuters report.
He added that the US move was "absolutely unfriendly," but reiterated Moscow's hope for improvements to the stressed relations between the two countries.
The announcement of US sanctions caused the value of the rouble to plunge early on Thursday and Russian stock markets to fall.
Ms 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.
The US is trying to play up this "anti-Russian topic as a way to continue demonising Russia" and make it appear that it is not fulfilling its international obligations, Ms Zakharova said.
The US sanctions came despite President Donald Trump's efforts to improve relations with Russia and its leader, Vladimir Putin, and his harsh criticism of the investigation into Russian interference in the 2016 election.
Mr Putin's spokesman Dmitry Peskov said the US move runs contrary to a "constructive" atmosphere at the Trump-Putin summit last month, and he strongly denied any Russian role in the poisoning in Britain.
"In our view, these and earlier restrictions are absolutely unlawful and don't conform to international law," Mr Peskov said.
Those sanctions will include the presumed denial of export licenses for Russia to purchase many items with national security implications, according to a senior State Department official who briefed reporters on condition of anonymity as he was not authorised to do so by name.
The US made a similar determination in February when it found that North Korea used a chemical weapon to assassinate North Korean leader Kim Jong-un's half-brother at the airport in Kuala Lumpur, Malaysia, in 2017.
Skripal and his daughter were poisoned by the Novichok military-grade nerve agent in the English city of Salisbury in March. Both eventually recovered. Britain has accused Russia of being behind the attack, which the Kremlin vehemently denies.
Months later, two residents of a nearby town with no ties to Russia were also poisoned by the deadly toxin. Police believe the couple accidentally found a bottle containing Novichok. One of them died.