US officials have told Bernie Sanders, the frontrunner for the Democratic presidential nomination, that Russia is trying to help his campaign, according to reports.

The Washington Post, citing people familiar with the matter, said the democratic socialist had been told his campaign was being targetted as part of an effort to interfere with the Democratic contest.

Mr Sanders released a statement calling Russian president Vladimir Putin an “autocratic thug”.

He said: “Unlike Donald Trump, I do not consider Vladimir Putin a good friend.

“He is an autocratic thug who is attempting to destroy democracy and crush dissent in Russia. Let’s be clear, the Russians want to undermine American democracy by dividing us up and, unlike the current president, I stand firmly against their efforts, and any other foreign power that wants to interfere in our election.”

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

The claims emerged a day after it was revealed that intelligence officials had briefed congress on fresh attempts by Moscow to interfere to help Mr Trump win re-election – apparently prompting an angry reaction from the president, who is said to fear his political opponents will use the assessment against him.

Mr Trump has repeatedly denied claims by his intelligence agencies that Russia sought to interfere in the 2016 election to help him. In 2018 he even said he believed Mr Putin’s denials over his own officials when the two men appeared at a news conference in Helsinki.

Some commentators have argued that Moscow’s real goal is to sow division and undermine the idea of democracy rather than to back one specific candidate.

Federal investigators charged 13 Russians in 2018 over a covert social media campaign that prosecutors said was aimed at dividing public opinion on controversial social issues as well as propping up Mr Sanders and then-candidate Mr Trump while also denigrating Hillary Clinton, the eventual Democratic nominee.

According to the indictment, those behind the campaign circulated an outline of themes for future social media content, with instructions to “use any opportunity to criticise Hillary and the rest (except Sanders and Trump – we support them)”.

On Friday Mr Trump told supporters at a rally in Las Vegas that he heard Democrats were trying to “start a rumour ... that Putin wants to make sure I get elected”.

“Listen to this, so doesn’t he want to see who the Democrat is going to be? Wouldn’t he rather have, let’s say Bernie?”

The president mentioned that Mr Sanders and his wife Jane spent their honeymoon in the Soviet Union in 1988.