CIA sources in Russia have reportedly gone quiet ahead of November's midterm elections, leaving intelligence agencies largely in the dark about Moscow's plans to interfere.

The New York Times reports that a number of CIA informants in Russia have grown silent as of late, likely as a result of aggressive counterintelligence efforts by the Kremlin.

Further increasing the difficulty of contacting U.S. sources is the size of the CIA's staff in Moscow, which has diminished as a result of Russia removing some U.S. personnel from the country in a tit-for-tat with President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE.

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“The Russians kicked out a whole bunch of our people,” John Sipher, a 20-year CIA veteran, told the Times. “Our station in Moscow is probably really small now and they are under incredible surveillance.”

The Kremlin, he added, had shown increasing willingness to silence critics and informants.

“The Russians are very focused and upset,” he added. “They have shown they are willing to kill sources.”

Russian election interference efforts in 2016 were exposed in the months following the election of Trump, who has repeatedly cast doubt on Moscow's involvement.

National security adviser John Bolton said that he warned his Russian counterpart Thursday during a meeting about the dangers of meddling in future U.S. political activities.

"I made it clear that we wouldn’t tolerate meddling in 2018, and that we were prepared to take necessary steps to prevent it from happening," Bolton said.

Missouri Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE, considered one of the most vulnerable Democrats up for reelection in November, reported last month that her campaign had been the target of attempted Russian intrusions.

“Russia continues to engage in cyber warfare against our democracy,” McCaskill said in a statement at the time. “I will continue to speak out and press to hold them accountable."

A report from The Times indicated that another Democratic lawmaker up for reelection has been targeted by Russian hackers so far in 2018, but the identity of the candidate has not been released.