US Democratic presidential contender Bernie Sanders has condemned Russia for its reported attempts to help his campaign, telling it to “stay out of American elections”, according to the report on Saturday.

“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,” the BBC quoted Sanders as saying while speaking in Bakersfield, California, on Friday.

The Vermont Senator said that US officials had informed him last month about Russian efforts to aid his campaign, but adding that it was not clear how Moscow intended to interfere.

The veteran politician also denounced Russian President Vladimir Putin as an “autocratic thug” whose government has “used internet propaganda to sow division in our country”.

Sanders, 78, is trailed closely by another progressive, Senator Elizabeth Warren of Massachusetts, who registered 17 per cent in the survey.

Last year, Sanders released a decade of tax returns, providing new insight into how the Democratic socialist from Vermont became a millionaire between his two presidential runs.

Earlier, according to the returns provided by his campaign, Sanders and wife Jane’s bottom line jumped from $240,622 in 2015, the year he launched his first White House bid, to $1,073,333 a year later, as the once-obscure lawmaker became a political sensation on the left and a bestselling author with royalties pouring in, CNN reported.

Sanders reported a total 2018 family income of $566,421 — $382,920 of which came from writing and royalties. The documents showed he paid $137,573 in federal taxes in 2018 and owed $8,267 in taxes for the year.

The poll is good news for Sanders, who increased his standing by 5 percentage points since the last Des Moines Register poll in November 2019. That survey showed Buttigieg surging to the front of the pack with 25-per cent support.

In 2016, during his first presidential campaign, Sanders released only one year of records — from 2014.

(With inputs from agency)