The sanctions, approved by Congress last week, seek to penalise Moscow for its alleged meddling in the 2016 US presidential election and Russia's annexation of Crimea.
US President Donald Trump grudgingly signed into law new sanctions against Russia on Wednesday, a move Moscow said amounted to a full-scale trade war and an end to hopes for better ties with the Trump administration.
The US Congress voted last week by overwhelming margins for sanctions to punish the Russian government over the alleged interference in the 2016 presidential election, annexation of Ukraine's Crimea and other perceived violations of international norms.
Trump signed the legislation behind closed doors and away from the cameras, after failed efforts to scupper or water down the bill.
Trump's reluctance was on full display in an angry signing statement, in which he called the legislation "significantly flawed."
"In its haste to pass this legislation, the Congress included a number of clearly unconstitutional provisions," he said, including curbs on the president's ability to conduct foreign policy.
The legislation, which also includes measures against North Korea and Iran, targets the Russian energy sector, giving Washington the ability to sanction companies involved in developing Russian pipelines, and placing curbs on some Russian weapons exporters.
It also notably constrains Trump's ability to waive the penalties, a statement of mistrust from the Republican controlled Congress which remains unsettled by Trump's warm words for President Vladimir Putin.
Russian Prime Minister Dmitry Medvedev called the sanctions tantamount to a "full-scale trade war," adding in a Facebook post that they showed the Trump administration had demonstrated "utter powerlessness."
"The hope that our relations with the new American administration would improve is finished," he wrote.
Trump's litany of concerns about the sanctions, which also affect Iran and North Korea, nonetheless raised the question of how vigorously Trump will enforce them and pursue action against Russia.
"While I favor tough measures to punish and deter aggressive and destabilizing behavior by Iran, North Korea, and Russia, this legislation is significantly flawed," Trump said in a message to lawmakers.
TRT World's Jon Brain has more details from Washington DC.
EU ready to retaliate against US sanctions
In response to the US sanctions, the European Union said it was ready if needed to retaliate within days against the new sanctions but believes Washington wants to spare European energy firms from any fallout.
"In the wake of Donald Trump signing off on stricter US sanctions against Russia, the Commission President Jean-Claude Juncker expressed his satisfaction, in principle, over the softening of the bill after the EU had expressed its concerns," said the commission.
"Moreover, US Congress has now also committed that sanctions will only be applied after the country's allies are consulted. And I do believe we are still allies of the US," Juncker was quoted as saying in the statement.
However, he warned Brussels was ready to retaliate if the sanctions in the end hurt EU energy firms.
"If the US sanctions specifically disadvantage EU companies trading with Russia in the energy sector the EU is prepared to take appropriate steps in response within days," the commission said.
To date, the sanctions have been coordinated on both sides of the Atlantic to maintain a united front.