Retaliatory measures recently levelled by the United States against Russia over alleged hacking activity are not the end of the story, Defence Secretary Ashton Carter said Tuesday.
"Some responses have been made. I think you should regard that as a start and not the end," Carter said at his last press conference at the Pentagon. He provided no further details.
Earlier Tuesday, Vyacheslav Volodin, the speaker of Russia's lower house of parliament, said in comments carried by state media that the new US sanctions against individual Russians were a manifestation of the "hysterics" of departing President Barack Obama and his administration.
Fellow lawmaker Andrei Lugovoi, who was one of the five Russians targeted in the latest extension of US sanctions, said the measure was absurd.
Lugovoi, a prime suspect in the murder of a former KGB agent in Britain, Alexander Litvinenko, joked that he might as well be accused of using performance-enhancing drugs in the 2014 Winter Olympics in Sochi.
The new sanctions also included Alexander Bastrykin, who heads Russia's federal Investigative Committee, an equivalent to the US FBI.
Last month, Obama expelled 35 Russian diplomats on allegations that Russian hackers had infiltrated US Democratic Party computers to influence November's presidential election.
Russian President Vladimir Putin announced at the time that he would not take any reciprocal measures.
Republican Donald Trump, who has publicly praised Putin and vowed to rebuild US-Russian relations, becomes president on January 20.
A US intelligence assessment released Friday said Putin had "ordered" a campaign to influence the US presidential election to undermine faith in the democratic process and help Trump.