Democratic presidential candidate Bernie Sanders has warned Russia to stay out of the 2020 presidential elections after US officials had told him Moscow was trying to aid his campaign.

"The intelligence community is telling us they are interfering in this campaign, right now, in 2020. And what I say to Mr Putin, if elected president, trust me you are not going to be interfering in American elections," Sanders told reporters on Friday in Bakersfield, California.

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Sanders, 78, a self-described democratic socialist senator from Vermont, is considered the frontrunner for the Democratic nomination and is favoured to win the Nevada caucuses on Saturday.

The Washington Post on Friday, citing people familiar with the matter, said US officials had told Sanders about the Russian effort and had also informed Republican President Donald Trump and US legislators.

It was not clear what form the Russian assistance took, the paper said.

Sanders said he was briefed about a month ago. His campaign noted the briefing was classified.

"We were told that Russia, maybe other countries, are going to get involved in this campaign," Sanders told reporters. "Look, here is the message: To Russia, stay out of American elections."

"What they are doing, by the way, the ugly thing that they are doing - and I've seen some of their tweets and stuff - is they try to divide us up," he said. "They are trying to cause chaos. They're trying to cause hatred in America."

Moscow denial

The Kremlin on Friday denied Russia was interfering in the US presidential campaign to boost Trump's re-election chances, following reports that American intelligence officials warned Congress about the election threat last week.

"These are more paranoid announcements which, to our regret, will multiply as we get closer to the (US) election," Kremlin spokesman Dmitry Peskov told reporters. "They have nothing to do with the truth."

Facebook declined to comment on whether it has seen any evidence of Russian assistance to Sanders's campaign. Last October, the company took down Russian-backed accounts that pretended to be from political battleground states.

Some of those accounts used Instagram to praise Sanders. Another used the #BlackLivesMatter hashtag and faulted Biden on race issues.

US intelligence agencies concluded that the Kremlin used disinformation operations, cyber-attacks and other methods in its 2016 operation in an effort to boost Trump in his presidential run, an allegation that Russia denies.

Trump, sensitive to doubts over the legitimacy of his win, has also questioned that finding and repeatedly criticised US intelligence agencies.

Russia's alleged interference sparked a two-year-long US investigation headed by Special Counsel Robert Mueller.

Mueller found no conclusive evidence of coordination between Russia and the Trump campaign. He also pointed at 10 instances in which Trump may have attempted to obstruct his investigation, as Democrats alleged, but left any finding of obstruction to Congress.

Trump slams Democrats

Of the eight contenders still seeking the Democratic presidential nomination, Sanders leads in polling in Nevada and nationally by about a dozen points over second-place Joe Biden.

Senator Elizabeth Warren, whose campaign has stagnated, hopes her standout performance in Wednesday's Democratic debate in Las Vegas - where she went on the offensive against billionaire newcomer and former mayor of New York Michael Bloomberg over his company's use of non-disclosure agreements in cases of sexual harassment - will earn some respect from Nevadans.

On Friday, Trump rallied his supporters in Las Vegas, where he attacked the "sick" and "radical socialist" contenders for his job.

Bloomberg was left "gasping for breath" after his debate debacle, Trump said, to loud cheers.

Sanders is "crazy," Warren is "a mess," Biden has been "angry" and billionaire activist Tom Steyer is a "schmuck," the president added.

Trump nonetheless sought to sow doubt in the ballot.

"I heard their computers are all messed up just like Iowa," he told his rally, citing no evidence.

"They say they're going to have a lot of problems tomorrow."