U.S. officials have warned Sen. Bernie Sanders (I-VT) that Russia is attempting to boost his prospects of winning the White House in 2020, sources familiar with the matter told the Washington Post.

President Donald Trump and Congressional lawmakers have also received briefings regarding the Kremlin’s efforts to help Sanders, reports the Post. In a statement to the newspaper, Sanders called Russian President Vladimir Putin a “thug” and said he stands “firmly against” any efforts to interfere in U.S. elections

“I don’t care, frankly, who Putin wants to be president,” Sanders said. “My message to Putin is clear: stay out of American elections, and as president I will make sure that you do.”

“In 2016, Russia used internet propaganda to sow division in our country, and my understanding is that they are doing it again in 2020. Some of the ugly stuff on the Internet attributed to our campaign may well not be coming from real supporters,” he added.

A spokesperson for the Sanders campaign would not comment on the nature of Russia’s attempted efforts to help the Vermont senator.

Speaking to reporters about the Post’s report, Sanders revealed that he learned of Russia’s interference a month ago and seemingly suggested that their efforts were made public Friday in a bid to hurt him one day before the Nevada caucuses.

The development comes after Sanders raised the prospect of Russia being behind some online posts in support of the lawmaker, which have been described as toxic and violent by former Mayor Pete Buttigieg (D) at this week’s Democrat debate in Nevada. Several other Sanders detractors have made similar claims.

“All of us remember 2016 and what we remember is efforts by Russians and others to try to interfere in our election and divide us up,” Sanders said, referring to memes posted by alleged Russian government agents in the last election.

Earlier this week, The New York Times reported that U.S. intelligence officials briefed lawmakers on Capitol Hill that Russia has “developed a preference” for President Trump to win re-election. The president took to Twitter Friday to rebuke the Times‘ report, tweeting: “Another misinformation campaign is being launched by Democrats in Congress saying that Russia prefers me to any of the Do Nothing Democrat candidates who still have been unable to, after two weeks, count their votes in Iowa. Hoax number 7!”

Another misinformation campaign is being launched by Democrats in Congress saying that Russia prefers me to any of the Do Nothing Democrat candidates who still have been unable to, after two weeks, count their votes in Iowa. Hoax number 7! — Donald J. Trump (@realDonaldTrump) February 21, 2020

In February 2018, then-special counsel Robert Mueller’s office criminally charged 13 Russians and three Russian entities for interfering in the 2016 election. The 37-page indictment alleged that the Russians’ activities “included supporting the presidential campaign of then-candidate Donald J Trump … and disparaging Hillary Clinton,” his failed Democrat White House rival. The 2016 Trump campaign denied colluding with Russia and Rod Rosenstein, then-deputy attorney general, said that the indictment made “no allegation… that any American had any knowledge” of Russia’s efforts. In March 2019, the Mueller investigation concluded that no criminal conspiracy occurred between the Trump campaign and Moscow.